Question: From what I understand, taking a polygon and breaking it up into composite triangles is called "tesselation". What's the opposite process called and can anyone link me to an algorithm for it?

Essentially, I have a list of 2D triangles and I need an algorithm to recombine them into a polygon.

Thanks!

Answer: 
I think you need to transform your triangles as a [half edge data structure](http://www.flipcode.com/archives/The_Half-Edge_Data_Structure.shtml), and then you should be able to easily find the half edges which have no opposite.

![alt text](https://i.stack.imgur.com/hSe3U.png)
The thing that you are calling tessellation is actually called [triangulation](http://en.wikipedia.org/wiki/Polygon_triangulation). The thing you are searching for is [tessellation](http://en.wikipedia.org/wiki/Tessellation) (you may have heard of it referred to as tiling).

If you are more specific about the problem you are trying to solve (e.g. do you know the shape of the final polygon?) I can try to recommend some more specific algorithms.