Four Terms

The fallacy of four terms (quaternio terminorum) occurs when four rather than three categorical terms are used in a standard-form syllogism.

Example:

All rivers have banks. All banks have vaults. So, all rivers have vaults.

The word "banks" occurs as two distinct terms, namely river bank and financial bank, so this example also is an equivocation. Without an equivocation, the four term fallacy is trivially invalid.