to be silent: to stop speaking, or to keep a matter unspoken
This verb describes the act of not speaking: a person who stops talking, holds back, and says nothing. In the Gospels and Acts it is what a crowd does when it has no reply left (Luke 20:26), what the disciples do after the Transfiguration when they tell no one what they have seen (Luke 9:36), and what a speaker is asked to do so that another can be heard (Acts 15:12-13; 1 Corinthians 14:28-34).
The same verb can also describe a truth kept unspoken. In Romans 16:25 the gospel is a mystery that was "kept secret" through long ages, held in silence until God chose to make it known. Here the silence is not a person pausing but a matter deliberately concealed, and then at last told.
Greek has several near neighbors for silence, and they divide along a fine line. The hush of stillness, a quiet that simply falls, belongs more to σιωπάω; the quiet of settling down and resting, ceasing from activity, is ἡσυχάζω; and being silenced by force, muzzled, is φιμόω. This word sits closest to the plain "hold one's peace": choosing not to speak.