Frequency is the number of times a person sees your message. Many advertising theorists note a marked decrease in direct response to online advertisements after three impressions per user, but no one's done a thorough audit of how frequency impacts brand awareness. Of course, this is extremely measurable online, so we can look forward to such data in the future. An advertiser trying to gain quick awareness will often specify that frequency be upped to enormous levels. Ever hear the same radio ad five times a day for a week? That's high frequency.