to ward off attack from; guard against assault or injury (usually fol. by from or against )

The sentry defended the gate against sudden attack.

to maintain by argument, evidence, etc.; uphold

She defended her claim successfully.

to contest (a legal charge, claim, etc.).

Law . to serve as attorney for (a defendant)

He has defended some of the most notorious criminals.

to support (an argument, theory, etc.) in the face of criticism; prove the validity of (a dissertation, thesis, or the like) by answering arguments and questions put by a committee of specialists.

to attempt to retain (a championship title, position, etc.), as in a competition against a challenger.

Law . to enter or make a defense.

noun

supporter , protector , sponsor , guardian , advocate , asserter , champion , defendant , patron , pleader , upholder , vindicator