Q.What is meant by "full employment"?

Full employment is an economic ideal rather than something ever fully achieved in practice — it describes the elimination of involuntary joblessness.

Define it, and note that a small amount of frictional unemployment still remains even at "full employment."

Full employment is the theoretical situation in an economy where every person who is willing and able to work at the prevailing wage rate actually finds work, so that involuntary unemployment is eliminated. In practice, no real economy achieves absolute full employment — some frictional unemployment (people temporarily between jobs) is normal and unavoidable even in a healthy labour market. "Full employment" is therefore best understood as the state where involuntary unemployment has been reduced to this minimal, unavoidable level, not as a literal zero-unemployment condition.