Jonathan Rosenberg has now offered a definition in a new Internet-Draft titled "What is a Session Initiation Protocol (SIP) Trunk Anyway?" Here is the abstract:
The term "Session Initiation Protocol (SIP) Trunk" has become almost commonplace amongst vendors and SIP providers. Even though the notion of a 'trunk' has a well defined meaning in circuit switched systems, it has never been defined for SIP. This document provides a formal definition for a SIP trunk, discusses its scope and applications, and establishes best practices for identification and security of SIP trunks.
The document makes for good reading even if you are not overly familiar with the concepts behind SIP trunks. Jonathan is looking for feedback and there will I'm sure be continued discussion on this topic.