Reach

Reach is the number of unique people who saw an ad at least once, as opposed to the total number of times it was shown.

Reach answers "how many different people did this touch," while impressions answers "how many times did this get shown, counting repeats." A campaign with 1 million impressions and 200,000 reach has an average frequency of 5 — the same 200,000 people saw the ad five times each, on average.

For awareness campaigns, reach at a reasonable frequency (enough repetition to register, not so much it becomes annoying or drives ad fatigue) usually matters more than raw impression volume.