If you think that a group of people secretly did something evil, that idea is a "conspiracy theory".
Some examples of conspiracy theories include:
- aliens have visited Earth but the world leaders are keeping it a secret
- drug companies already have cures to many diseases but they keep them hidden so that they can keep making money
The phrase "conspiracy theory" sounds negative. So if someone says that one of your ideas is a conspiracy theory, you might disagree:
A: That's just a silly conspiracy theory.
B: No it's not! It's a proven fact! Look it up.