What is the definition for the prefix 'il'?

The prefix il-, as well as the similar prefixes im-, in-, and ir-, all mean "not", or "the opposite of".

They have their roots in the Latin in- (which also means "not" or "the opposite of"), but the -n- was assimilated with the sound of the following consonant, to become Words such as

illegal: the opposite of legal

infamous: the opposite of famous

impossible: the opposite of impossible

irresponsible: the opposite of responsible

These four prefixes share the same definition, and the same roots, and are worth mentioning together.