Impression
One counted ad delivery or display. It need not have been viewable, and nobody has to have seen or clicked it.
See it
What it is
An impression is one counted delivery of your ad: the platform served it into a page, a feed, or an app slot and logged it. No click required, no reading required, and no promise it ever reached a pair of eyes. It is the denominator under almost every other ad number you look at: CTR, CPM, and frequency all start here.
The word people confuse it with is reach. Reach counts people, impressions count showings, and impressions divided by reach gives you frequency (the average number of times one person saw the ad). 30,000 impressions against 6,000 people is a frequency of 5, which is often the moment a campaign starts to annoy its own audience.
Gotcha: not all impressions were seen. A served impression means the ad was delivered, while a viewable impression means enough of it was actually on screen long enough to be seen (the industry rule of thumb is 50% of pixels for one second for display, two seconds for video). Reporting that quietly bills served impressions while you assume viewable ones is how cheap inventory stays cheap.
Ask AI for it
Build a small visual explainer that separates impressions, reach, and frequency. Render a 10x10 grid of 100 person icons representing an audience: fill 40 of them to show reach, and stack small numbered dots on some of those filled icons to show repeat views, with a running total of impressions above. Add a frequency readout computed as impressions divided by reach, and a slider that raises impressions so the viewer watches reach flatten while frequency climbs. Muted palette, one accent color for the filled icons, labels in small caps.