A2[C or U]the cause of an event or situation or something that provides an excuse or explanation: The reason for the disaster was engine failure, not human error.[+ question word]The reason why grass is green was a mystery to the little boy.[+ (that)]The reason (that) I'm ringing is to ask a favour.not standardThe reason I walked out was because I was bored.[+ to infinitive]The police have (every good) reason to believe that he is guilty.She was furious, and with reason (= with good cause).For some reason/For reasons best known to himself(= for reasons no one else knows about) he's decided to leave his job.Motives and reasonsReasons and explanationsby reason offormal›because of: He's always asked to these occasions by reason of his position.Connecting words which introduce a cause or reason