Hypothesis

A statement that could be true, which might then be tested.

Example: Sam has a hypothesis that "large dogs are better at catching tennis balls than small dogs".
We can test that hypothesis by having hundreds of dogs of different sizes try to catch tennis balls.

Sometimes the hypothesis won't be tested, it is simply a good explanation (which could be wrong). Conjecture is a better word for this.

Example: we notice the temperature drops just as the sun rises. Our hypothesis is that the sun warms the air high above us, which rises up, and then cooler air comes from the sides.

Note: when someone says "I have a theory" they should say "I have a hypothesis", because in mathematics a theory is actually well proven.