Hypothesis- a proposed scientific explanation for a set of observations

So basically, a hypothesis is an educated guess of what you believe will happen.

Example: you are conducting an experiment on the effect of light on plant growth. You have two plants: one on a window sill of a well-lit room, and one in a box with no light. Your hypothesis could be "The plant on the window sill will grow more than the one in the box." or "Light effects the growth of plants."