A person or business that is advertised; the subject or object of advertising.
From advertise (from Old French 'advertir' meaning 'to warn' or 'direct attention') plus -ee (suffix denoting a person who receives an action). This modern coinage mirrors the pattern of employee, payee, and other -ee words.
Advertisee is a perfect example of how English creates words for roles we didn't have names for—in our modern economy, companies and people are increasingly the 'advertisees' that social media platforms use to attract actual customers.
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