Etymology
The word pantheism comes from two words: pan is a Greek word meaning "all" (also used as , whereas theism means "a belief in god(s)", from the Greek word theos (which also had a meaning of "divine").

Definition
Putting the parts together, we get the meaning "the universe is divine". Not divine as in "a supernatural being who created, sustains, and rules the universe", but as in "inspiring a deep reverence and/or awe".

The physical universe all around us is, in my opinion, so more mysterious and awe-inspiring than any of the many deities man has created for himself throughout our history. There have been so many men who have devoted their entire lives to figuring out the mysteries of just one of this universe's many mysteries, and have remained in awe by the universe throughout their entire lives. Many people have built up an entire spirituality around this awe and/or inspiration of the physical universe.