Definition of "Software Specification"

- A specification describes WHAT a program does, a design describes HOW the program does it, and documentation describes WHY it does it
- A specification is a document which forms a contract between the customer and the designer.
- A specification is a document by which we can judge the correctness of an implementation.

A specification is a precise, unambiguous and complete statement of the requirements of a system (or program or process), written in such a way that it can be used to predict how the system will behave.

Reference:
http://scom.hud.ac.uk/scomtlm/book/node8.html