Full Idea
An explanation, I think, is an account of etiology: it tells us something about how an event was caused.
Clarification
Greek 'aitia' means 'cause'
Gist of Idea
An explanation tells us how an event was caused
Source
David Lewis (On the Plurality of Worlds [1986], 1.5)
Book Reference
Lewis,David: 'On the Plurality of Worlds' [Blackwell 2001], p.73
A Reaction
Will this cover mathematical explanations? Numbers would have to have causal powers.