What is unit test

Simple and clear checklist from The Art of Unit Testing: With Examples in .Net.
A unit test should have the following properties:

• It should be automated and repeatable.
• It should by easy to implement.
• Once it's written, it should remain for future use.
• Anyone should be able to run it.
• It should run at the push of a button.
• It should run quickly.

Source: The Art of Unit Testing: With Examples in .Net