Unemployment due to individuals searching for jobs.
(Peter Dungan, Toronto PPG1002H and Mankiw, N. Gregory, Ronald Kneebone, Kenneth J. McKenzie and Nicholas Rowe. 2008. Principles of Macroeconomics, 4th Canadian ed. Toronto: Thomson Nelson.)
Some workers are always going to be in the process of moving from one job to another or looking for their first job. Unemployment due to these periods of job search is “frictional” unemployment. Some level of frictional unemployment is consistent with an economy being considered to be at a "full employment" level.