A scam is a sneaky or dishonest plan that's meant to con someone. A diet scam, for example, falsely claims that people will lose weight if they buy special pills or drinks.
A scam is basically a hoax dressed up to look like a real business plan, worthwhile invention, or investment idea. Scams are almost always designed to make money for their creators. You can also use the word as a verb: "His evil plan was to scam people into buying fake tickets to the baseball game." Word experts know that scam came from US carnival slang, but beyond that its origins are a mystery.