Someone who runs across a public place with no clothes on, usually as a joke, a dare, or a way to get attention. The classic scene is a sports match: a person jumps the fence, runs naked across the pitch while the crowd laughs, and security officers chase them off. That is what most people picture when they hear the word. The tone is important. "Streaker" is a light, half-amused word — newspapers use it, commentators use it, and it does not suggest a sex crime; it suggests silliness and showing off. That said, it is still against the law in most countries, and a streaker at a big match will normally be arrested and fined afterwards. The verb is "to streak" (he streaked across the field), the action is "streaking". The word became common in the 1970s, when streaking was a craze on American university campuses, and it still carries a slight flavour of that time. You are far more likely to read it than to say it.