verb /ɛkˈspleɪn,ɪkˈspleɪn/
a) To give a sufficiently detailed report about (a) the reason for something, about why something happened, about a causal chain of events; about (b) how something works, about how elements in a system interact; about (c) how to do something, about the steps which need to be accomplished in order to accomplish a certain goal.
b) To give a valid excuse for some past behavior.
Wikipedia foundation.