illegal

meaning in plain English

Against the law; not allowed by the rules.

Picture a sneaky kid stealing a bike—his act is illegal, like breaking the rulebook of the playground.

for example

It’s illegal to drive without a license in most places.

Something is illegal when the government says you can’t do it and will punish you if you do. Ever wonder why you can’t just take whatever you want from a store without paying? Because that would be illegal, and the store could call the police.