The guru is considered essential and to the Buddhist devotee, the guru is the "enlightened teacher and ritual master", states Stephen Berkwitz. The guru is known as the "vajra guru" (literally "diamond guru"). Initiations or ritual empowerments are necessary before the student is permitted to practice a particular tantra, in Vajrayana Buddhist sects found in Tibet and South Asia. The tantras state that the guru is equivalent to Buddha, states Berkwitz, and is a figure to worship and whose instructions should never be violated. There are Four Kinds of Lama (Guru) or spiritual teacher (Tib. lama nampa shyi) in Tibetan Buddhism:   In various Buddhist traditions, there are equivalent words for "guru", which include "Shastri" (teacher), "Kalyana Mitra" (friendly guide, Pali: Kalyāṇa-mittatā), "Acarya" (master), and "Vajra-Acarya" (hierophant). The "guru" is literally understood as "weighty", states Alex Wayman, and it refers to the Buddhist tendency to increase the weight of canons and scriptures with their spiritual studies. In Mahayana Buddhism, a term for Buddha is Bhaisajya guru, which refers to "medicine guru", or "a doctor who cures suffering with the medicine of his teachings". In Sikhism, "Guru" is the source of all knowledge which is Almighty. In Chopai Sahib, Guru Gobind Singh states about who is the Guru:  The Sikh gurus were fundamental to the Sikh religion, however the concept in Sikhism differs from other usages. Sikhism is derived from the Sanskrit word "shishya", or disciple and is all about the relationship between the teacher and a student. The concept of Guru in Sikhism stands on two pillars i.e. Miri-Piri. 'Piri' means spiritual authority and 'Miri' means temporal authority. Traditionally, the concept of Guru is considered central in Sikhism, and its main scripture is prefixed as a "Guru", called Guru Granth Sahib, the words therein called "Gurbani". As an alternative to established religions, some people in Europe and the USA looked to spiritual guides and gurus from India and other countries. Gurus from many denominations traveled to Western Europe and the US and established followings. In particular during the 1960s and 1970s many gurus acquired groups of young followers in Western Europe and the USA.