Let's say I have a terrain mesh of some triangles, and several smaller meshes which I want to add to that terrain mesh (think ground and some small mounds or berms for example). What would I look for to find algorithms to let me do this? I know it's like some kind of computational geometry but I wonder if anyone might know search terms or even better urls to look at. Ideally I would like to do this on the fly to simplify drawing of a terrain where the basic ground and the detailed "interesting" bits are stored separately. Right now we draw the ground and the other parts separately which is less than ideal.
I think the term you are look for is CSG, or constructive solid geometry. There are basically three different operations: boolean union, boolean intersection, and boolean difference. You need the boolean union to merge two overlapping meshes into one. I'm sure you'll be able find some useful information with those terms.
Some links to get you started: