Impression

An impression simply means a showing of your advertisement. Just because you create an advertisement doesn’t guarantee you will get shown. You need to have other aspects of your account set up correctly and there must be searches relevant to your keyword for you to get an impression. This is a major way that AdWords differs from buying a print ad in which your advertisement would be included in every single copy of the magazine, regardless of the reader’s interest in what you do.

Read Google’s definition.