Guru principle
Teaching principle of God
There are various aspects to the Supreme God Principle. These various aspects of God perform specific functions in the Universe. This is pretty much akin to the government of any country which has various departments to facilitate the smooth governing and functioning of the country as a whole.
Just as we have a department for education in a government, which facilitates teaching of modern sciences throughout the country, the aspect of God that looks after spiritual teaching and spiritual growth in the Universe is known as the Guru. This is known as the unseen or unmanifest (nirguṇ) Guru or the Teaching Principle of God.