TITLE: Math Symbol for "Where"
QUESTION [15 upvotes]: Pretty much any math text I've read introduces notation through a similar format of equation-notation or notation-equation form e.g

ax+b+c
  where a = ..., b = ..., etc.

or 

Let a = ..., b = ..., etc.
  ax+b+c

I tend to see the former more and was wondering if there has been an official symbol for "where" to introduce notation?  If not, why not?

REPLY [14 votes]: In his famous paper How to write mathematics, P.R. Halmos says the following about "where"

"Where" is usually a sign of a lazy afterthought that should have been thought through before. "If $n$ is sufficiently large, then $|a_n| < \varepsilon$, where 
  $\varepsilon$ is a preassigned positive number"; 

That being said it is common to write 

$x = 2^n$, where $n$ is some positive integer, 

although I would prefer 

$x = 2^n$ for some positive integer $n$.

Now to answer your question, there is no specific symbol for "where" that I know of.