Definition
The testing of individual components, to demonstrate that each performs as described in its specification. Usually carried out by the component’s developer. Black box test and white box test techniques apply.
[Source: BCS]
Comment
When performed code problems can be identified early in the development lifecycle before the components are linked together. Where there are faults, the problem area is known at once. The tests can ensure that the component performs as specified, that all code written is tested and that the component is fit to be included in a larger system.
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.