Hypothesis
Etymology
From Greek "hypothesis" (a placing under, a foundation) — "hypo-" (under) plus "thesis" (a placing, a proposition). A hypothesis is what you put underneath an inquiry to build on, a foundation held only as long as it bears the weight of the evidence.
noun · /hy-POTH-uh-sis/
A proposed, testable explanation put forward to be checked against evidence.
What it names
Use it for a provisional guess with a job to do — a claim specific enough that observation could knock it down.
Example
Their hypothesis — that the plants grew faster under blue light — was something they could actually test by Friday.
Related idea
It sits below theory: a hypothesis is a single tentative claim awaiting test; a theory is a broad, well-supported framework that has survived many tests. Calling a hunch a "theory" and a mature theory "just a hypothesis" both blur this ladder.