A first-pass set of definitions, in increasing order of cynicism:
Scrum is a methodology designed to help an organisation deliver a product every few weeks rather than every few years.
Scrum is a set of rituals intended to prevent motivated, clever people from becoming cowboy coders. It channels agile behaviour into a predictable cycle of deliverables at a given level of quality.
Scrum is a set of processes designed to be adopted by organisations who want to find out if their staff can ‘do’ agile.
Scrum is a form of project management where neither the start nor the end of the project are defined.
What definitions have I missed out?