Definition

The process of exercising software to verify that it satisfies specified requirements and to detect errors.

[Source: BCS]

Comment

Effective testing finds critical bugs quickly and efficiently, and in a manner in which they can be reproduced.

Effective testing reduces the risk to the company that the system will fail in a significant fashion. Testing is a large response to a very large risk. The earlier that testing is introduced to the system, the cheaper that the defects found will be to fix