ad hoc
The Latin phrase ad hoc translates as “toward this”, and is used in mathematics to describe anything that has been made up specifically for one particular purpose.
Examples

1.
an ad hoc definition is a temporary definition that might be useful for the discussion at hand. For instance, let a nice set be a closed set with smooth boundary.

2.
If a proof makes use of an ad hoc construction, the proof (typically) does not make use of a general theory for the problem.
See also http://en.wikipedia.org/wiki/Ad_hocArticle on ad hoc on Wikipedia
Title  ad hoc 

Canonical name  AdHoc 
Date of creation  20130322 14:45:20 
Last modified on  20130322 14:45:20 
Owner  mathcam (2727) 
Last modified by  mathcam (2727) 
Numerical id  9 
Author  mathcam (2727) 
Entry type  Definition 
Classification  msc 0001 