In advertising, reach is the amount of people who have interacted with an ad or a campaign. Reach is sometimes confused with impressions. A reach of 1,000 people means this amount of people saw the ad. On the other hand, 1,000 impressions indicates how many times the ad was displayed, no matter how many unique people saw it.
In a broader sense, reach may mean the number of individuals or households that have been exposed to a given media channel within a set period of time.