That which causes something: an efficient cause, a proximate cause.
The reason this tree fell is that it had rotted.
A motive for an action or a determination.
The reason I robbed the bank was that I needed the money.
An excuse: a thought or a consideration offered in support of a determination or an opinion; that which is offered or accepted as an explanation.
(Can we verify(+) this sense?) proof, more or less decisive, for an opinion or a conclusion