A scientific theory, as defined by Kevin Padian in his testimony in the Kitzmiller v. Dover Area School District trial, is
"A theory, in science, [...] means a very large body of information that's withstood a lot of testing. It probably consists of a number of different hypotheses, many different lines of evidence. And it's something that is very difficult to slay with an ugly fact, as Huxley once put it, because it's just a complex body of work that's been worked on through time."
In common parlance, "theory" is roughly equivalent to "hunch" or "guess". But in science, a theory is a well-supported explanation. The common usage of the word "theory" is closer in meaning to the scientific use of the word "hypothesis". In science, however, a hypothesis is an idea about something that seems probable, while a theory has been tested and is supported by evidence.