The Collaborative International Dictionary of English v.0.48:
Balancer \Bal"an*cer\, n. 1. One who balances, or uses a balance. [1913 Webster] 2. (Zool.) In Diptera, the rudimentary posterior wing. [1913 Webster]WordNet (r) 3.0 (2006):
balancer n 1: an acrobat who balances himself in difficult positions 2: either of the rudimentary hind wings of dipterous insects; used for maintaining equilibrium during flight [syn: halter, haltere, balancer]