Sanskrit, from सद्गुरु sat "real, true," and गुरु guru "teacher." A term used in various Hindu traditions to refer to the highest teacher, a person who has accomplished self-realization and is totally pure. The significant term here is Sat, a reference to the Absolute, the primordial, incomprehensible origin of everything.
"Brahman is Sat, the Absolute, Reality. That which exists in the past, present and future; which has no beginning, middle and end; which is unchanging and not conditioned by time, space and causation; which exists during the waking, dream and deep sleep states; which is of the nature of one homogeneous essence, is Sat. This is found in Brahman, the Absolute." —Swami Sivananda
"A true Sat Guru is absolutely free from passion, anger, selfishness, greed, hatred and egoism. He is beyond worldly temptations. He is able to remove the ignorance of people. He can clear any kind of doubt. He can give practical, easy lessons to control the mind and the senses. His instructions are very impressive. Even an arrogant proud man bends his head before him. He is able to take the students to the door of Moksha. He can remove the obstacles, pitfalls and snares on the path. He is versed in the Vedas. He is sinless. He is an ocean of mercy. He is a friend to all... The Satguru is Brahman [the Absolute] Himself. He is an ocean of bliss, knowledge and mercy. He is the captain of your soul. He is the fountain of joy. He removes all your troubles, sorrows and obstacles. He shows you the right divine path. He tears your veil of ignorance. He makes you immortal and divine. He transmutes your lower, diabolical nature. He gives you the rope of knowledge and saves you when you are drowning in this ocean of Samsara. Do not consider him to be only a man. If you take him as a man, you are a beast." —Swami Sivananda
"...blessed be the one who knocks at the thirteenth door, the door of the Ain, because that one will become a sat-guru, that is, a living manifestation of the Absolute, here and now, that one will become a seity beyond good and evil... the most authentic manifestation of the Absolute in a human body: a God, like Jesus of Nazareth (a paramarthasattya)..." —Samael Aun Weor, The Dimensions of the Tree of Life
"Truly it is said that the Satguru is God Incarnate. His greatness is indescribable... Satguru is a prophet and a messenger of God, is a cosmic friend and benefactor of the world. He is a guiding star to the suffering mankind." —Swami Sivananda
See also: What is the Absolute?