This question is in the reverse direction of a common MO question. Instead of being faced with a formal definition and asking for some intuition for the definition, I have a concept with I understand intuitively but where I do not know a definition.

I have been in interested in the proofs of the 4-colour theorem which uses the notion of a "discharge rule". This evening I was discussing discharge rules with a speaker who was not familiar with the proofs of the 4-colour theorem and was embarrassed that I could not give a definition. I have seen examples of discharge rules and I feel I would know one if I saw one. However I have no recollection of reading a definition. Did I overlook a definition in one of the papers? and if not, could someone complete the following sentence:

**Definition** A *discharge rule* is ...