balancer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that balances.

from The Century Dictionary.

• noun One who balances or weighs; a weigher of things in or as in a balance.
• noun An acrobat; one who balances himself.
• noun One who or that which keeps a thing or things in equilibrium; that which maintains or helps to maintain something in a state of balance or equipoise.
• noun Specifically, in entomology, a halter (which see); a poiser; the small organ supposed to be useful in balancing the body; one of a pair of slender processes with clubbed ends placed near the insertion of the wings, especially of dipterous insects.
• noun In herpetology, an elongate cylindrical rod protruding from each side of the head of larval salamanders, in front of the gills: permanently retained in certain forms, as the cæcilias and some salamanders.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who balances, or uses a balance.
• noun (Zoöl.) In Diptera, the rudimentary posterior wing.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun One who, or that which, balances.
• noun The rear wings of certain insects used for stability in flight.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an acrobat who balances himself in difficult positions
• noun either of the rudimentary hind wings of dipterous insects; used for maintaining equilibrium during flight