An impression refers to the count of the total number of times a specific piece of content, such as an advertisement, post, or a webpage, is displayed, regardless of whether it was clicked or not. Impressions are a key metric used to measure the reach or visibility of a piece of content.

For example, if a user sees an advertisement on a webpage, that counts as one impression. If the same advertisement is displayed to the user again on a different page, that would count as a second impression.

Impressions are a valuable metric in understanding user engagement and the effectiveness of content or advertising campaigns. They provide insight into how often a piece of content is being displayed, which can be used to gauge visibility and reach.

In the context of Statsig, impressions can be used to monitor the rollout of a product or feature. For instance, watching usage/impressions start to ramp up in a specific region or for a new feature. They can also be used to watch for spikes in error logs, understand which users are being affected and when, and monitor the rollout of a fix as errors return to baseline.

It's important to note that impressions do not provide information about user interaction with the content. An impression is counted whenever the content is displayed, regardless of whether the user interacts with it or not. For more detailed engagement metrics, other measures such as clicks, conversions, or time spent on page might be used in conjunction with impressions.