A hypothesis is an educated guess that can be tested or proven wrong. You make a hypothesis when you think about what might happen if certain things are true.
When people use the word "hypothesize", they usually mean making a prediction based on some information they have, but not necessarily having all the facts yet. For example, a scientist might hypothesize that a new medicine will work well for a certain disease after studying its effects on animals and in lab tests. They don't say it works until they've tested it thoroughly and gathered more data.
To believe or assert on uncertain grounds.
