"an act of explaining; a meaning or interpretation assigned," late 14c., explanacioun, from Latin explanationem (nominative explanatio) "an explanation, interpretation," noun of action from past-participle stem of explanare "to make plain or clear, explain," literally "make level, flatten," from ex "out" (see ex-) + planus "flat" (from PIE root *pele- (2) "flat; to spread").
An individual fact is said to be explained, by pointing out its cause, that is, by stating the law or laws of causation of which its production is an instance. Thus. a conflagration is explained, when it is proved to have arisen from a spark falling into the midst of a heap of combustibles. [J.S. Mill, "Logic"]