Impressions
The number of times a post was displayed on someone's screen, regardless of whether they engaged with it. The most basic reach metric on every major social platform.
Impressions count the number of times a post was rendered on someone's screen. A single user scrolling past your post counts as one impression. The same user seeing it again later counts as a second impression. It is a measure of distribution, not engagement.
Impressions sit at the top of the social metrics funnel. From impressions you derive reach (unique users who saw the post), engagement rate (engagements / impressions), and click-through rate (link clicks / impressions). Without impressions data, you can't reason about whether a post failed because it had bad distribution or bad content.
Different platforms count impressions slightly differently. X counts an impression any time the tweet appears in a user's feed, search results, or profile, even if it doesn't fully scroll into view. LinkedIn counts an impression when the post becomes at least 50% visible on screen for at least 300 milliseconds. Instagram is closer to LinkedIn's definition. The differences are technical but matter when comparing platforms.
A common mistake is to conflate impressions with attention. A post can get 100,000 impressions and zero meaningful attention if it scrolled past everyone in half a second. This is why dwell time (on LinkedIn) and watch time (on video platforms) matter as secondary metrics.
For most creators, impressions is the primary growth metric to watch. Engagement follows distribution; if your impressions are flat or falling week over week, no amount of clever copy or design will compensate.