Kenneth G. Wilson (1923). The Columbia Guide to Standard American English. 1993.
hypothesis (n.)
The plural is hypotheses, pronounced hei-PAHTH-uh-seez or hi-PAHTH-uh-seez. Be wary: using hypotheses as a singular is Substandard, probably a hypercorrection by someone unsure, and it will be harshly judged by Standard speakers. See FOREIGN PLURALS.