1 forming verbs and their derivatives: a down, away (descend; deduct). b completely (declare; denude; deride).
2 added to verbs and their derivatives to form verbs and nouns implying removal or reversal (decentralize; de-ice; demoralization).
Etymology: from or after L de (adv. & prep.) = off, from: sense 2 through OF des- f. L dis-
Useful english dictionary. 2012.