Lying in court about anything anything is called perjury. More here:
Perjury - Criminal Law Lawyer Source (http://www.criminal-law-lawyer-source.com/terms/perjury.html)

Its called Perjury... and Perjury can result in jail time.

Yep, the above answers are right: If you lie under oath in court, then if you are caught lying (no matter how insignificant you think that the lie was) then you have been caught perjuring yourself. In many areas that can result in jail time.