Definitions of senior:

• noun:   an undergraduate student during the year preceding graduation
• noun:   a person who is older than you are
• adjective:   older; higher in rank; longer in length of tenure or service

  Example: "Senior officer"

• adjective:   advanced in years; (`aged' is pronounced as two syllables)

  Example: "Senior citizen"

• name:  A surname (rare: 1 in 100000 families; popularity rank in the U.S.: #11454)