Literally, Sunnah refers to a way or habit. However, in Islamic terminology, it refers to the Sayings and Acts of the Holy Prophet , including the times in which he remained silent, which is also referred to as Taqreer. Taqreer is included in Sunnah because it is the duty of a Prophet to speak and act if something haram/illegal is done in his presence. So, if Prophet Muhammad remained silent in some matters, it means that they were acceptable to him, and hence, to other Muslims as well.