What is Frictional Unemployment?

Legal Definition
Frictional unemployment is the unemployment that results from time spent between jobs when a worker is searching for, or transitioning from one job to another. It is sometimes called search unemployment and can be based on the circumstances of the individual.
-- Wikipedia
Legal Definition
Measurable time out-of-work, like a contractor who is between contracts. Also, the time for an employee laid off and seeking other work. Temporary unemployment due to time lags job hunting if already out of a job.