Summary

The same word is used with two different meanings.

Examples

1. Criminal actions are illegal, and all murder trials are criminal actions, thus all murder trials are illegal. (Here the term "criminal actions" is used with two different meanings. Example borrowed from Copi.)
2. The sign said "fine for parking here", and since it was fine, I parked there.
3. All child-murderers are inhuman, thus, no child-murderer is human. (From Barker, p. 164; this is called "illicit obversion")
4. A plane is a carpenter's tool, and the Boeing 737 is a plane, hence the Boeing 737 is a carpenter's tool. (Example borrowed from Davis, p. 58)

Proof

Identify the word which is used twice, then show that a definition which is appropriate for one use of the word would not be appropriate for the second use.