WordNet (r) 3.0 (2006):

sour orange
    n 1: any of various common orange trees yielding sour or bitter
         fruit; used as grafting stock [syn: sour orange, Seville
         orange, bitter orange, bitter orange tree, bigarade,
         marmalade orange, Citrus aurantium]
    2: highly acidic orange used especially in marmalade [syn:
       bitter orange, Seville orange, sour orange]