'Declare' is Latin for 'make completely clear' — from 'clarus' (bright), kin to 'clarity' and 'exclaim.'
To make known formally, officially, or explicitly; to state emphatically; to announce one's position or intention.
From Middle English 'declaren,' from Old French 'declarer,' from Latin 'dēclārāre' (to make clear, to reveal, to declare), from 'dē-' (completely, from top to bottom) + 'clārāre' (to make clear), from 'clārus' (clear, bright, famous). The PIE root is *kelh₁- (to shout, to call), which also gave rise to English 'clear,' 'clarify,' 'claim,' and 'exclaim.' The original sense was to make
The Latin 'clārus' meant simultaneously 'clear' (in sound or vision) and 'famous' — to be famous was to be clearly seen. This double meaning survives: a 'clarion' call is both loud and clear, and 'Clare' as a name means 'bright' or 'famous.' The Declaration of Independence is, etymologically, 'a making-completely-clear of independence.'