explanation
Reasons why something is true, given to someone who already accepts that it is.
An explanation takes a fact as already accepted and says why or how it came about. An argument tries to convince someone that a claim is true. The same words can do either job; the deciding factor is the passage's purpose for its audience.
Examples
"The milk spoiled because the fridge broke" explains an accepted fact.
"The fridge must have broken, because the milk spoiled" argues for a claim.
See also