JeffG:
A set of policies and procedure's, supported by a specific equipment configuration, to help safely conduct a dive objective.
Jeff is pretty succinct, and usually dead-on, but I think there is a bit more to the explanation.
"Policies and procedures" suggest a comprehensive and cohesive set of written guidance that covers all possibilities. But, there is a large universe of possibilities...
DIR is more of a paradigm or perspective than a set of hard, defined policies and procedures. It is a way of thinking about diving that purports to be effective for all (non-commercial) diving, globally. There are certainly performance requirements and equipment specifications that define DIR as well - e.g. "in the absence of <fill in the blank: level trim, only 5 D-rings, etc.> it isn't DIR".
But rather than a set of specific written guidance that purports to be effective for every dive environment under ALL circumstances, its more of a team-oriented, holistic philosophical approach to diving - requirements to keep fit and work on endurance; requirements to consider diet and exercise; requirements to 'think situations all the way through' in terms of dive planning and identifying resource requirements in terms of people, training, equipment, as well as mindset. It offers numerous principles that apply more broadly to how divers should approach their diving: e.g. "if you don't need it, don't take it" (on any specific dive); or "we use specific defined gasses because it keeps logistics more efficient, provides for redundancy across the team, and ensures all team members run the same deco schedule".
People focus on the equipment because thats most tangible; but DIR-at-large is not about equipment or how to perform an S-Drill. It is more ambiguous than that, because the universe of environments to which it (claims to apply) is so broad. Its overall goal is to make diving safer and more enjoyable.
FWIW. YMMV.
