Pantheism is the belief that God and the universe are equivalent (the same thing.) A pantheist believes that everything that exists is a part of God or that God is a part of everything that exists. The name pantheism comes from the words theism (belief in God) and pan (all).
Any doctrine or philosophy that believes that the universe and everything in it is God is said to be pantheistic. Most pantheists believe the universe is sacred and the earth and nature are divine. Most of the early Greek philosophers from Thales on to Aristotle believed in some sort of pantheism.