Theory: A tested hypothesis that is supported by evidence.

Conspiracy: In criminal law, a conspiracy is an agreement between two or more people to commit a crime at some time in the future. Criminal law in some countries or for some conspiracies may require that at least one overt act be undertaken in furtherance of that agreement, to constitute an offense.

So, a tested hypothesis supported by evidence of an agreement between two or more people to commit a crime.

And the english-speaking public has been brainwashed into thinking these words mean something completely different when combined.