Wordnet 3.0
NOUN (1)
1. 
 someone who takes the place of another (as when things get dangerous or difficult); 
- Example: "the star had a stand-in for dangerous scenes"- Example: "we need extra employees for summer fill-ins"[syn: stand-in, substitute, relief, reliever, backup, backup man, fill-in]
WordNet (r) 3.0 (2006):
fill-in
    n 1: someone who takes the place of another (as when things get
         dangerous or difficult); "the star had a stand-in for
         dangerous scenes"; "we need extra employees for summer
         fill-ins" [syn: stand-in, substitute, relief,
         reliever, backup, backup man, fill-in]