We define an impression as an appearance on a user’s screen. It’s the number of times someone put eyeballs on your content.
Impressions count the number of appearances, not the number of individual people who see your content. For example, if one person sees your content one time, that’s one impression. But if that same person sees your content five times, that’s five impressions.
Note: Impressions do not measure the number of times your Tweet may have been screenshotted and embedded in other content. It also doesn’t count if someone views a text preview of your Tweet or if they see your Tweet on a third-party platform.