Impressions
Also called impression count · thumbnail impressions
The number of times YouTube showed a video's thumbnail to a viewer, whether or not that viewer clicked it.
In more detail
Impressions is a reach number, not an engagement number: it counts a thumbnail being placed in front of someone on the home feed, in search, or in a suggested panel, before any decision about clicking has been made. YouTube Studio reports it alongside click-through rate precisely because the two answer different questions: how far a video was shown, against how often that showing worked.
Example
A video with 200,000 impressions and a 2% click-through rate earned about 4,000 views from that traffic. The same 4,000 views from only 40,000 impressions would mean a 10% rate, a much stronger result even though the view count looks identical.
Why it matters
Impressions on their own say nothing about quality; a video can rack up impressions from being pushed to the wrong audience and still fail on click-through. Reading impressions and click-through rate together is what tells you whether a video's problem is reach or packaging, and those call for different fixes.