1. Older, elder; especially used after a person's name to denote the elder of two bearing the same name in a family; also (after a simple surname) the elder of two boys of the same surname in a school, etc.: Stuart's father was known as John Jones, Senior; often abbreviated John Jones, Sr.  

At school, there were two boys with the surname Smith; they were known as Smith, Senior and Smith, Junior.
2. A term for a very mature person; especially, someone who is past the age of retirement: Mrs. Thompson, the senior who lives in the same apartment building that Jane does, is very active and goes to the gym at least three times a week."
3. Someone ranking before others as a result of longer service or tenure in a position; that is, superior to others in standing or status: Greg's position at the bank was senior to all the other employees since he had worked at the bank for more than 40 years.

The senior class is the last year of an educational program; such as, high school or college: Mary's son, Ted, was a senior in his last year at high school and because of the expense, she was not sure whether he would be able to go on to a university after graduating.