You'd use a " suit coat." A sports coat is a little less formal.

你会穿一件“西装外套”。运动外套不太正式。

I told her the coat suited her, but she looked very embarrassed, saying that it was a cheap one she had bought a long time ago.

我对她说，这件衣服真合身。没想到她看上去很不好意思，解释说这件外衣是很早以前买的，而且很便宜。