What impressions are: viewings of a webpage or ad, counted by your tracking software.
What impressions are not: the number of people looking at your webpage or ad. (Those are called visitors.)
Why we care about impressions: We use the number of impressions to your page, ad, or keyword to calculate things like click-through rate (CTR).
This post is part of an ongoing Clickipedia series, defining all the terms you need to know for conversion rate optimization. Keep checking back each week to learn more, and don’t forget to check out the whole series.