special case

Short definition from Wiktionary

Noun

1. 1
   A specialized instance of a more general phenomenon, category, etc.
2. 2
   A specialized instance of a more general phenomenon, category, etc: A class of phenomena that is a subset of a more general class.
3. 3
   A specialized instance of a more general phenomenon, category, etc: A theorem or other statement that follows directly from a more general statement.

Verb

1. 1
   To treat something as a special case; to handle in an explicitly different way.
2. 2
   Alternative form of special case (“treat specially”).