It has many different uses in U.S. English. Literally, the word means "old" or "older than" someone else. It has many other mean related meanings. 1) The four years of high school and/or college each have four traditional names: freshman year, sophomore year, junior year, senior year. The students in their last year are "seniors." High school seniors traditionally attend a social event called the "senior prom." 2) Old people are politely called "senior citizens" or "seniors." I'm a senior myself. When I'm trying to find out the price of a theatre ticket or a museum admission, I'll ask "do you have a senior discount," a lower price for people over 65. 3) In an organization, it can mean relative rank. Corporations may use the word in job titles: "Software engineer I, Software engineer II, Senior software engineer, Principal software engineer, Software architect." 4) In an organization, it can also refer to length of tenure within the organization; the "senior" in a group is the person who's been their longest. If I've been here seven years and he's only been here five years, "I'm senior to him." Some organizations put great stress on "seniority" when deciding who will receive a promotion. 5) If a man's and his son have the same name, while they are both alive they will use words "Senior" and "Junior" after their names, abbreviated "Sr." and "Jr." A famous entertainer was known through most of his career as "Sammy Davis, Jr." His less-famous father was "Sammy Davis, Sr." After the father died, the son was known simply as "Sammy Davis."

As far as I know, that's how the last year of High School is called in the United States.

You can be a senior at school, meaning you are in the class of older children. You can be a senior citizen, which in my country is anyone who is over 60 years of age.