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