This is the 4365th most frequent Japanese word.
反則
"反則" means "foul" or "violation of rules."
反則 is used here to refer to a rule violation or foul in a sports context.
彼は試合で反則をしたので罰を受けることになった。
He committed a foul in the match, so he will face penalties.
反則 is metaphorically used to describe an unfair act not necessarily related to rules but societal expectations.
この製品をこんな低価格で売るのは反則でしょう。
Selling this product at such a low price is unfair, isn't it?
反則 is used figuratively here to imply something being so exceptional it feels like a 'rule-breaking' advantage.