A sound you make out loud, written down. It is a breathy, whistling out-breath, and English writes it "whew" (some writers spell it "phew"). It carries relief above all: the danger has passed, the test is over, the near-miss missed. "Whew! I thought I'd lost my passport." It also does two other jobs. It can mean you are physically worn out or too hot — "Whew, it's boiling in here" — because that is the same tired breath. And it can mean plain surprise at something big, expensive, or impressive: "Whew, that's a lot of money." Which one it means comes almost entirely from the situation, so a reader has to look at what happened just before. It sits alone, usually with a comma or exclamation mark after it, at the start of what you say. It is casual and spoken — natural in speech, in messages, in dialogue in a story, but out of place in an essay or a business email.