[C] an idea or opinion of what something is likeI didn't get much of an impression of the place because it was dark when we drove through it.
What was your impression of Charlotte's husband?
I don't tend to trust first impressions (= the opinion you form when you meet someone or see something for the first time).
[+ that] When I first met him I got/had the impression that he was a shy sort of a bloke.
be under the impression to think or understand a particular stated thingI was under the impression (that) you didn't get on too well.
He was under the mistaken (= false) impression (that) you were married.