Entry 1 sense Webster, 1913 Singletree · Sin·gle·tree · IPA /ˈsɪŋɡəlˌtɹiː/ 01 n. The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree. 1. The pivoted or swinging bar to which the traces of a harnessed horse are fixed; a whiffletree.