Sufism, or Taṣawwuf (الْتَّصَوُّف‎), variously defined as "Islamic mysticism", "the inward dimension of Islam" or "the phenomenon of mysticism within Islam", is mysticism in Islam, "characterized ... [by particular] values, ritual practices, doctrines and institutions" which began very early in Islamic history and represents "the main manifestation and the most important and central crystallization of" mystical practice in Islam. Practitioners of Sufism have been referred to as "Sufis" (from صُوفِيّ‎ ṣūfiyy / ṣūfī). Historically, Sufis have often belonged to different ṭuruq or "orders" – congregations formed around a grand master referred to as a wali who traces a direct chain of successive teachers back to the Prophet Muhammad.