Definition
The testing of an individual unit of code, to demonstrate that it performs as described in its specification. Black box test and White box test techniques apply. Unit testing is normally carried out by the development team.
[Source: BCS]
Comment
When performed, code problems can be identified early in the development lifecycle before the units are linked together. Where there are faults, the problem area is known at once. The tests can ensure that the units perform as specified, and that all code written is tested.
On many programmes and large projects, testing will take up a significant portion of the budget. But it will receive less management attention than an activity of this size warrants.