Undistributed Middle

In syllogistic logic, failing to distribute the middle term over at least one of the other terms is the fallacy of undistributed middle. Also called the fallacy of maldistributed middle.

Example:

All collies are animals.
All dogs are animals.
Therefore, all collies are dogs.

The middle term ("animals") is in the predicate of both universal affirmative premises and therefore is undistributed. This formal fallacy has the logical form: All C are A. All D are A. Therefore, all C are D.