Also called the Politician's Syllogism or Equivocation. A standard three-step syllogism uses three terms -- the things that are being linked by the line of reasoning. If A, then B. If B, then C. Therefore if A, then C. The fallacy of four terms occurs when, exactly like it says four terms are used instead of three. In most cases, a single term (B) is used two (or more) times, in differing contexts with different meanings; and yet the argument treats the two usages as exactly the same, since the same term was used. It's best explained by this example from Land of the Blind.