Wordnet 3.0
ADJECTIVE (1)
1. 
 used of the fourth and final year in United States high school or college; 
- Example: "the senior prom"[syn: senior(a), fourth-year]
The Collaborative International Dictionary of English v.0.48:
fourth-year \fourth-year\ adj.
   of or pertaining to the fourth and final year in a U. S. high
   school or college.
   Syn: senior(prenominal).
        [WordNet 1.5]
WordNet (r) 3.0 (2006):
fourth-year
    adj 1: used of the fourth and final year in United States high
           school or college; "the senior prom" [syn: senior(a),
           fourth-year]