idea/opinion

1an idea, a feeling or an opinion that you get about somebody/something, or that somebody/something gives youa general/an overall impressionan initial/a lasting impressionimpression (of somebody/something)to geta good/bad impressionof somebody/somethingMyfirst impressionof him was favourable.Shegives the impressionof being very busy.impression (that…)I did notget the impressionthat they were unhappy about the situation.My impression isthat there are still a lot of problems.Try and smile. You don't want togive people the wrong impression(= that you are not friendly).

effect

2the effect that an experience or a person has on somebody/somethingabig impressionimpression (on somebody)His trip to Indiamade astrongimpressionon him.My wordsmade no impressionon her.You'll have to play better than that if you really want tomake an impression(= to make people admire you).

drawing

3a drawing showing what a person looks like or what a place or a building will look like in the futureThis is anartist's impressionof the new stadium.

false appearance

5an appearance that may be falseClever lighting creates an impression of space in a room.

mark

6a mark that is left when an object is pressed hard into a surfaceThe dentist made an impression of his teeth.
Idioms
(be) under the impression that…
believing, usually wrongly, that something is true or is happening
I was under the impression that the work had already been completed.The soldiers scattered, under the impression that it was an enemy attack.
Usage note:thinkbelievefeelreckonbe under the impressionThese words all mean to have an idea that something is true or possible or to have a particular opinion about somebody/something.think to have an idea that something is true or possible, although you are not completely certain; to have a particular opinion about somebody/something:Do you think (that) they'll come?Well, I like it. What do you think?believe to have an idea that something is true or possible, although you are not completely certain; to have a particular opinion about somebody/something:Police believe (that) the man may be armed.think or believe?When you are expressing an idea that you have or that somebody has of what is true or possible,believe is more formal thanthink. It is used especially for talking about ideas that other people have;think is used more often for talking about your own ideas:Police believe…I think… When you are expressing an opinion,believe is stronger thanthink and is used especially for matters of principle;think is used more for practical matters or matters of personal taste.feel to have a particular opinion about something that has happened or about what you/somebody ought to do:We all felt (that) we were unlucky to lose.reckon (informal) to think that something is true or possible:I reckon (that) I'm going to get that job.be under the impression that… to have an idea that something is true:I was under the impression that the work had already been completed.to think/believe/feel/reckon/be under the impressionthat…It is thought/believed/reckonedthat…tobe thought/believed/felt/reckonedto be somethingto think/believe/feel somethingabout somebody/somethingtosincerely/honestly/seriously/mistakenly think/believe/feel