Wordnet 3.0
ADJECTIVE (1)
1. 
 being in such a position that top and bottom are reversed; 
- Example: "a quotation mark is sometimes called an inverted comma"- Example: "an upside-down cake"[syn: inverted, upside-down]
The Collaborative International Dictionary of English v.0.48:
Upside-down \Up"side-down`\, a.
   having the part normally pointed upward pointed downward;
   inverted.
   [PJC]
WordNet (r) 3.0 (2006):
upside-down
    adj 1: being in such a position that top and bottom are
           reversed; "a quotation mark is sometimes called an
           inverted comma"; "an upside-down cake" [syn: inverted,
           upside-down]