A wrong answer or incorrect answer is a question answered incorrectly during gameplay on Who Wants To Be A Millionaire? When a contestant gives an answer which happens to be incorrect, he/she would lose some money and his/her prize would fall back to the previous guaranteed prize (on the original British version, either £1,000 or £32,000).
Providing an incorrect answer on a 'free shot' question, (that is, the first question after a guaranteed sum of money has been achieved) will have the same effect as a contestant walking away with a particular cash prize (meaning no money can be lost on that particular question).
Answering the first question or any question before to the first guaranteed prize will leave the contestant with no money whatsoever. Very rarely contestants have walked away before reaching the first guaranteed prize to avoid losing all of their money already won.