You failed high school biology?
In science, a theory is:
In science, a body of descriptions of knowledge is usually only called a theory once it has a firm empirical basis, i.e., it
1. is consistent with pre-existing theory to the extent that the pre-existing theory was experimentally verified, though it will often show pre-existing theory to be wrong in an exact sense,
2. is supported by many strands of evidence rather than a single foundation, ensuring that it probably is a good approximation if not totally correct,
3. makes predictions that might someday be used to disprove the theory,
4. is tentative, correctable and dynamic, in allowing for changes to be made as new data is discovered, rather than asserting certainty, and
5. is the most parsimonious explanation, sparing in proposed entities or explanations, commonly referred to as passing Ockham's razor.
This is true of such established theories as special and general relativity, quantum mechanics, plate tectonics, evolution, etc. Theories considered scientific meet at least most, but ideally all, of the above criteria. The fewer which are matched, the less scientific it is; those that meet only several or none at all, cannot be said to be scientific in any meaningful sense of the word.