declaration (n.)

late 14c., declaracioun, "an explanation, a statement, action of stating clearly," from Old French declaration and directly from Latin declarationem (nominative declaratio) "a making clear or evident, a disclosure, exposition," noun of action from past-participle stem of declarare "make clear, reveal, disclose, announce," from de-, here probably an intensive prefix (see de-) + clarare "clarify," from clarus "clear" (see clear (adj.)).

The meaning "proclamation, formal public statement" is from c. 1400; that of "document by which an announcement or assertion is formally made" is from 1650s, as in declaration of independence, which is is recorded from 1776 (the one issued in that year by the British American colonies seems to be the first so called; though the phrase is not in the document itself, it was titled that from the first in the press). Declaration of war is by 1762.