Math Symbol for "Where" 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?

The use of natural language is often more effective when presenting an idea. In my opinion the less often a symbol is used where a few words can go, the better.
That said, there are many places where symbols are useful and simplify matters.
The word "where" can often be replaced with "such that", and corresponding to this we have a few regularly used symbols.
For instance, in set builder notation the colon (or bar) is used to indicate a condition (read "where" or "such that"):
$$\{ x \in \mathbb{R} : x > 0 \}.$$
Also $|$ and $\ni$ are used for this same purpose.
