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In the US a senior is a student who is in his 4th year of high school and about to graduate. It's common to take "senior photos" to remember the year and the subject will sometimes want to feature a favorite hobby, sport, interest, etc.. during the shoot.
edit: "senior" can also refer to someone in his or her 4th year of university but when talking about "senior pictures" people typically are referring to a senior in high school.
Aha. Thank you. We (UK) have 'senior citizens' - older people - and 'senior moments' e.g. going to fetch something, then forgetting what we went to fetch.