The number of unique users who click on ads. Uniqueness is based on each user's DoubleClick cookie ID.
For some reports, you can specify a time period for uniqueness. If you choose to view unique impressions Daily, Weekly, or Monthly, the same unique user can be counted once for each specified time period. For example, if you choose Daily, and a unique user clicks twice on Monday, three times on Tuesday, and once on Wednesday, that user is counted as 1 unique click for each day. If you choose Lifetime, each unique user is counted only once.
Similarly, for reports where no time period is specified, each unique user is counted only once.