If you want to see smoke come out of a scientist's ears (figuratively speaking, of course), tell him or her that evolution (or gravity, for that matter) is "just a theory." In casual conversation, a theory may be just an idea, but in science, it's a system of ideas that stands up to repeated challenges.
Some people may try to dismiss widely accepted theories of global warming and evolution as merely speculative. But while a theory can never be "proven" (because this is science!), it's far from mere speculation. A scientific theory may incorporate several related hypotheses, gradually gaining acceptance only after being tested and supported through reproducible observation and experimentation [source: Zimmerman].
Another concept closely associated with a theory is a scientific law. One simple way to remember the two is that a law explains what will happen; theories seek to explain why it happens. Laws can often be expressed as mathematical equations. For example, Newton's law of gravity predicts what will happen if we drop an object, but it doesn't tell us why it happens. For that, we use Einstein's theory of general relativity [source: Krampf].
The next word on our list can be a helpful tool for testing a theory ... but you probably use it to mean something else entirely.