Wordnet 3.0
ADJECTIVE (1)
1. 
 moderately formal; 
 requiring a dinner jacket; 
- Example: "he wore semiformal attire"- Example: "a black-tie dinner"[syn: semiformal, semi-formal, black-tie]
WordNet (r) 3.0 (2006):
semiformal
    adj 1: moderately formal; requiring a dinner jacket; "he wore
           semiformal attire"; "a black-tie dinner" [syn:
           semiformal, semi-formal, black-tie]