An impression is a term used in the context of online advertising to refer to a single instance of an ad being shown to a user. An impression is counted each time an ad is loaded on a user's device, regardless of whether the user actually sees or interacts with the ad. Impressions are often used as a measure of the reach or exposure of an advertising campaign, as they indicate how many users have potentially been exposed to the ad. Impressions are typically measured in the thousands, and are often used in conjunction with other metrics, such as click-through rate (CTR) or conversion rate, to evaluate the effectiveness of an ad or campaign.