Hypothesis
A hypothesis is a testable, tentative explanation for an observation that proposes how one variable affects another.
Definition
A hypothesis is a possible answer to a science question that you can test with an experiment, based on what you already know rather than a wild guess. It proposes a relationship between an independent and a dependent variable, often written in "if, then, because" form, and it must be specific enough that an experiment could show it to be false. In formal design it is paired with a null hypothesis (no effect), and the investigation is built to distinguish the two using controls, adequate trials, and analysis that accounts for measurement uncertainty. A hypothesis that survives repeated testing across many studies may contribute to a theory, but it never becomes one on its own.
Example
"If I give a bean plant more water, then it will grow taller, because plants need water to grow." "If the ramp angle increases from $10^\circ$ to $30^\circ$, then the cart's time to the bottom will decrease, because a steeper ramp gives a larger component of gravity along the ramp." Testing whether dissolved salt raises water's boiling point, a group measures $0$, $1$, $2$, and $3\ \text{mol/L}$ solutions three times each and finds means of $100.0$, $100.5$, $101.0$, and $101.5^\circ\text{C}$ with ranges of $0.2^\circ\text{C}$, a spread small enough to reject the null hypothesis.
Key Insight
A hypothesis is not a guess you hope is right; it is a claim you are willing to prove wrong. Data can support it or fail to support it, but they never prove it true forever, and one clear counterexample is enough to send it back for revision. The strength of a hypothesis lies in what it forbids: a statement that fits every possible result cannot be tested, so it is not scientific no matter how reasonable it sounds.