What is Impressions?
Impressions are the total number of times an ad or piece of content is displayed, regardless of whether it was clicked or seen by a unique person. A single person can generate many impressions if the same ad is shown to them repeatedly.
TL;DR
An impression is one ad display, counted every time it happens, even if the same person sees it five times.
Formula
Impressions = count of every individual ad display (unique viewers not required)
Why It Matters
Impressions are the foundation every other visibility metric is built on, since click-through rate, cost per impression, and frequency all depend on this raw count. On its own, though, a rising impression number can be misleading, since it can grow just from showing the same ad to the same people more often rather than actually reaching anyone new. That's why impressions are almost never evaluated alone, they need reach to show how many distinct people were exposed, and frequency to show how often each of them saw it. Ignoring that pairing risks reading a campaign as more successful than it actually is, when what really grew was just repetition to a fixed audience. Marketers use the combination to decide whether to expand targeting for new reach or cap frequency to avoid ad fatigue.
Example
If an ad is shown to the same person 3 separate times, that counts as 3 impressions but only 1 unit of reach. A campaign that serves 250,000 impressions over a week to 80,000 unique users has an average frequency of roughly 3.1, a figure calculated by dividing impressions by reach. Impressions alone say nothing about how many people were actually reached or how often, which is why the metric is almost always reported alongside reach and frequency rather than by itself.
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