means A proposed explanation for something, put forward on limited evidence so it can be tested and either supported or shot down.

from From Greek hypothesis, literally 'a placing under' or 'foundation' — from hypo- ('under') plus thesis ('a placing, a proposition'). The idea is of something set down underneath an argument as its base. It passed through Late Latin and into English around the late 1500s, first meaning a supposition laid down as groundwork, later narrowing to the scientist's testable proposition.