What is illegal defense in the NBA?

Answer 1

when you spend three seconds in the paint without being in arms length of an opposing player.

Answer 2

The one occurance of illegal defense in the NBA that I know about is the "3 seconds in the key" rule. If you're on defense and aren't aggresively guarding someone, you can't spend more than 3 seconds in the key at a time. This is a relatively new rule in the NBA, and it was made because zone defense is now legal in the NBA, but they wanted to have some restrictions on it (this rule). Before zone defense was legal, it was also "illegal defense."