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question based on many observations
It's an educated guess. It's not a question, it's the proposed answer to a question. And not just a specific empirical question, like "At what temperature does this substance melt?" but to a general theoretical question like "Why do substances melt at one particular temperature?"
The word comes from the Greek words hypo ("beneath") and thesis ("statement"), and it means that which underlies many satatements -- a basis, a foundation of thought.
Because in the philosophy of empiricism ("truth is only that which is measureable, and measured") a hypothesis is something that is inherently tentative, to some degree, because it can always be subject to further experimental test, the word "hypothetical" has entered our vocabulary as something that is tentative or even unlikely. ("Hypothetically, if I wanted to get rid of my science teacher, would you help me hide the body?") I think this has tended to cloud the general understanding of what a hypothesis is within the context of empirical science. It's NOT something that is tentative per se. It just means it's a general statement about causes and effects ("If I do X, then Y will always happen.") which is subject to experimental test.
Some hypotheses are extremely reliable, such as the Second Law of Thermodynamics, or the equivalence of inertial and gravitational mass. Others are indeed considered much more tentative.