What is a person called when they believe in a higher power but not a god?

Possibly the best term would be "pantheist". Pantheism is the philosophy that "God" and the Universe (Nature) are the same. Pantheists do not believe in a God in the common and traditional sense of a personal creator being. Some Pantheists recognize the intelligence involved in the rational and systematic functioning of Nature and the Universe, which is not necessarily intentional but is "minded" in a sense.