The definition of a singletree is a bar between the pulling animal and the wagon or plow that balances the weight being pulled.(noun)
An example of a singletree is a metal crossbar between a horse and a buggy.
See singletree in Webster's New World College Dictionary
Origin: altered (as if < single) < swingletree < ME swingle, rod, whip + tre, tree
See singletree in American Heritage Dictionary 4