(countable) An unproven conjecture.
I have a theory about who broke into the school last night, but I have no proof to back it up.
(uncountable) An expectation of what should happen, barring unforeseen circumstances.
So we’ll be there in three hours? — Yes, that’s the theory.
(countable) (sciences) A coherent statement or set of statements that attempts to explain observed phenomena.
There is now a well-developed theory of electrical charge.
(countable) (sciences) A logical structure that enables one to deduce the possible results of every experiment that falls within its purview.
The theory of relativity was proposed by Einstein.
(uncountable) (mathematics) A field of study attempting to exhaustively describe a particular class of constructs.
Knot theory classifies the mappings of a circle into 3-space.
(countable) (logic) A set of axioms together with all statements derivable from them.
A theory is consistent if it has a model.