It's certainly a map of some time, and if it's implemented using hashing then it's a hash table. I'm not sure whether the advantage of your structure over either having a map/hash table of vertex to list of triangles, and then either intersecting the results of doing three look-ups or scanning the result of one look-up to find matching triangles (if there only a few triangles for each vertex), or having one table for the vertex -> list of triangles mapping and another table of tuple(vertex,vertex,vertex) -> triangle mapping. Implementing custom maps can be efficient, but can also be complicated for the performance (space or speed) gained.