In economics, full employment has more than one meaning. The majority of economists believe the unemployment rate is greater than 0% when there is full employment, correspond to the technical term Non-Accelerating Inflation Rate of Unemployment (NAIRU). In common-sense terms, the rate of unemployement is difference between those actively seeking employment and the availability of matching employments. Since neither is the full workforce either fully engaged or actively seeking some employment nor are the requirementos of the employments necessarily matching the workers, what should practically be considered "full employment" is a matter of contention.
20th century British economist William Beveridge stated that an unemployment rate of 3% was full employment. Other economists have provided estimates between 2% & 7%, depending on the country, time period, and the various economists' political biases.
Specific Technical Viewpoints
Long Range Aggregate Spending
The concept of full employment has so far been used in conjunction with the long run LRAS curve, where long run potential output is also the full employment level of output. Full employment does not mean that there is 'zero unemployment', but rather that all of the people willing and able to work have jobs at the current wage rate. Full employment is the quantity of labour employed when the labour market is in equilibrium.
The following should be understood in discussions of NAIRU: Governments that follow it are attempting to keep unemployment at certain levels (usually over four percent, and as high as ten or more percent) by keeping interest rates high. As interest rates increase, more bankruptcies of individuals and businesses occur, meaning less money to hire staff or purchase goods (the making and distributing of which requires workers, which means jobs). It might also be noted that the main cause of inflation is not high employment, but rather the ability of banks to make money with little to no backing with things of value (commodities such as gold and silver are some examples), thus flooding the market with money and decreasing the value of each dollar already issued in the process, assuming the economy has not kept up to this increase in issued loans. Economists such as Milton Friedman  and Dr. Ravi Batra have theorized ways that a modern economy could have low inflation and near full employment (as in close to 100% of those who are not students and are healthy enough to work, and who wish to work at any given point in time), as of yet these have yet to be widely disseminated through the press or introduced by most governments. Paul Martin - former finance minister and past Prime Minister of Canada - once held that full employment could be achieved, yet let go of this idea after gaining power. For more on this see the expose "Shooting the Hippo" by Linda McQuaig, author and former columnist for many of Canada's top newspapers.
Friedman's view has prevailed so that in much of modern macroeconomics, full employment means the lowest level of unemployment that can be sustained given the structure of the economy. Using the terminology first introduced by James Tobin (following the lead of Franco Modigliani), this equals the Non-Accelerating Inflation Rate of Unemployment (NAIRU) when the real gross domestic product equals potential output. This concept is identical to the "natural" rate but reflects the fact that there is nothing "natural" about an economy.
At this level of unemployment, there is no unemployment above the level of the NAIRU. That is, at full employment there is no cyclical or deficient-demand unemployment. If the unemployment rate stays below this "natural" or "inflation threshold" level for several years, it is posited that inflation will accelerate, i.e. get worse and worse (in the absence of wage and price controls). Similarly, inflation will get better (decelerate) if unemployment rates exceed the NAIRU for a long time. The theory says that inflation does not rise or fall when the unemployment equals the "natural" rate. This is where the term NAIRU is derived.
The level of the NAIRU thus depends on the degree of "supply side" unemployment, i.e., joblessness that can't be abolished by high demand. This includes frictional, structural, classical, and Marxian unemployment.
Ideas associated with the Phillips curve questioned the possibility and value of full employment in a society: this theory suggests that full employment -- especially as defined normatively -- will be associated with positive inflation. The Phillips curve tells us also that there is no single unemployment number that one can single out as the "full employment" rate. Instead, there is a trade-off between unemployment and inflation: a government might choose to attain a lower unemployment rate but would pay for it with higher inflation rates. In 1968, Milton Friedman, leader of the monetarist school of economics, and Edmund Phelps posited a unique full employment rate of unemployment, what they called the "natural" rate of unemployment. But this is seen not as a normative choice as much as something we are stuck with, even if it is unknown. Rather than trying to attain full employment, Friedman argues that policy-makers should try to keep prices stable (a low or even a zero inflation rate). If this policy is sustained, he suggests that the economy will gravitate to the "natural" rate of unemployment automatically.
Some Economists estimate a "range" of possible unemployment rates. For example, in 1999, in the United States, the Organisation for Economic Co-operation and Development (OECD) gives an estimate of the "full-employment unemployment rate" of 4 to 6.4%. This is the estimated "structural" unemployment rate, (the unemployment when there is full employment), plus & minus, the standard error of the estimate. (Estimates for other countries are also available from the OECD.) 
An alternative, more normative, definition (used by some labor economists) would see "full employment" as the attainment of the ideal unemployment rate, where the types of unemployment that reflect labor-market inefficiency (such as structural unemployment) do not exist. Only some frictional unemployment would exist, where workers are temporarily searching for new jobs. For example, Lord William Beveridge defined "full employment" as where the number of unemployed workers equaled the number of job vacancies available. He preferred that the economy be kept above that full employment level in order to allow maximum economic production.
Long before Friedman and Phelps, Abba Lerner (1951) developed a version of the NAIRU. Unlike the current view, he saw a range of "full employment" unemployment rates. He distinguished between "high" full employment (the lowest sustainable unemployment under incomes policies) and "low" full employment (the lowest sustainable unemployment rate without these policies).
Whatever the definition of full employment, it is difficult to discover exactly what unemployment rate it corresponds to. In the United States, for example, the economy saw stable inflation despite low unemployment during the late 1990s, contradicting most economists' estimates of the NAIRU.
The idea that the full-employment unemployment rate (NAIRU) is not a unique number has been seen in recent empirical research. Staiger, Stock, and Watson found that the range of possible values of the NAIRU (from 4.3 to 7.3% unemployment) was too large to be useful to macroeconomic policy-makers. Robert Eisner suggested that for 1956-95 there was a zone from about 5% to about 10% unemployment between the low-unemployment realm of accelerating inflation and the high-unemployment realm of disinflation. In between, he found that inflation falls with falling unemployment.
Worse, the NAIRU doesn't stay the same over time -- and can change due to economic policy. For example, some economists argue that British Prime Minister Margaret Thatcher's anti-inflation policies using persistently high unemployment led to higher structural unemployment and a higher NAIRU.
The active pursuit of national full employment through interventionist government policies is associated with Keynesian economics and marked the postwar agenda of many Western nations, until the stagflation of the 1970s. Australia was the first country in the world in which full employment in a free society was made official policy by its government. On May 30, 1945, The Australian Labor Party Prime Minister John Curtin and his Employment Minister John Dedman proposed a white paper in the Australian House of Representatives titled [[Full Employment In Australia, the first time any government apart from totalitarian regimes had unequivocally committed itself to providing work for any person who was willing and able to work. Conditions of full employment lasted in Australia from 1941 to 1975.
- The OECD on measuring the NAIRU
- Devine, James. 2004. The "Natural" Rate of Unemployment. In Edward Fullbrook, ed., A Guide to What's Wrong with Economics, London, UK: Anthem Press, 126-32.
- Eisner, Robert. 1997. A New View of the NAIRU. In Paul Davidson and Jan A. Kregel, eds. Improving the Global Economy. Cheltenham, UK: Edgar Elgar, 1997.
- Friedman, Milton. 1968. The Role of Monetary Policy. American Economic Review. 58(1) March: 1-21.
- Lerner, Abba. 1951. Economics of Employment, New York: McGraw-Hill.
- Staiger, Douglas, James H. Stock, and Mark W. Watson. 1997. The NAIRU, Unemployment and Monetary Policy. Journal of Economic Perspectives. 11(1) Winter: 33-49.
- "A Monetary and Fiscal Framework for Economic Stability", 1948, American Economic Review, Vol. 38, No. 3 (Jun., 1948), pp. 245-264