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.

Elderly people; especially, those who are retired and living on pensions: Many of the senior citizens in Marian's urban area are active in social and political groups.

A secondary educational facility that typically consists of the highest grade levels of 10, 11, and 12 or 9, 10, 11, and 12: Shirley Jones was teaching English classes at the local senior high school which consisted of two 11th grade classes and three classes of 10th grade students.

A temporary lapse of memory as a person ripens in age: The couple in their golden years realized that they were often experiencing senior moments because they couldn't always remember some well-known word or name when they were talking to each other.

Rick Perry, a 65-year-old governor of Texas and a Republican candidate for President of the U.S. in 2011, apparently had a senior moment or "brain freeze" when he stopped in mid-sentence while struggling to remember the name of the Department of Energy as one of three federal agencies he had often said should be eliminated.

Perry's senior moment resulted in a pained look on his face as he stammered and started over again but he still couldn't remember what he wanted to say; so, he changed the subject after a 53 second senior moment.