A weight that balances another weight WordNet 3.0 & Open English WordNet

A counterweight is a weight that, by applying an opposite force, provides balance and stability of a mechanical system. Wikipedia

A force or influence equally counteracting another. OmegaWiki

Something of equal weight used to provide equilibrium; counterweight. Wiktionary

Something of equal weight used to provide equilibrium. Wiktionary (translation)

A weight that is put in opposition to an equal weight so it keeps that in balance. Wiktionary

An equal power or force acting in opposition; a force sufficient to balance another force. Wiktionary

A heavy mass of often iron or concrete, mechanically linked in opposition to a load which is to be raised and lowered, with the intent of reducing the amount of work which must be done to effect the raising and lowering. Counterweights are used, for example, in cable-hauled elevators and some kinds of movable bridges (e.g. a bascule bridge). Wiktionary

Heavy mass reducing the amount of work which must be done to effect the raising and lowering of something. Wiktionary (translation)

One who makes equal; a balancer. Wiktionary

A balancer. Wiktionary (translation)