Theories are a set of ideas that predict past and future events, such as the Theory of Gravity talking about how masses exert a magnetic pull on other masses and that this increases with size and proximity. Using this, we can predict that objects will still fall when they're here on earth.
A law is a theory or hypothesis that is always true and has been completely tested. If I recall correctly, there technically are no laws in science because not EVERY test has been run, even on gravity.
A definition is an explanation of how you are using a term. For example, you could say Feminist and mean two very different things: 1) someone who believes that there should be equal opportunity and wages regardless of an individual's gender, or 2) someone who believes that women are better than men. It just lets everyone know how to understand what you are saying when you use a particular term.
