How do scientists use the word theory differently than the average person?

The average person will generally use "theory" in the way a scientist would say "hypothesis." Hypotheses are unproven; theory is based on fact.

For example, germ theory is the explanation for diseases caused by bacteria, viruses, and parasites, theory of gravity explains the univeral attractive force between all matter, and the theory of evolution explains the diversity of life we find on earth. These are all supported a mountain of facts which are usually verified across several braches of science.