A syllogistic fallacy where the middle term (the term that appears in both premises) is not distributed (quantified on the left hand side of the logical expression) in either premise in which it occurs.
Abstractly, the fallacy looks like:
All A are C All B are C Therefore, all A are B
In plain English, it means an argument that goes like this:
All communists favor regulation All democrats favor regulation Therefore, all democrats are communists
Or:
All dogs are mortal Socrates is mortal Therefore, Socrates is a dog
See original on c2.com 