1. A hole or small recess for pigeons to nest
2. A small open compartment (as in a desk or cabinet) for keeping letters or documents
3. A neat category which usually fails to reflect actual complexities
4. A small compartment in a desk or case for the keeping of letters, documents, etc.

A numbered compartment in a post office where mail is put to be called for; SYN. POB, call box, letter box.