The word "Guru" is a Sanskrit word meaning teacher, honored person, religious person or saint. Sikhism though has a very specific definition of the word Guru. It means the descent of divine guidance to mankind provided through ten Enlightened Masters. This honour of being called a Sikh Guru applies only to the ten Gurus who founded the religion starting with Guru Nanak in 1469 and ending with Guru Gobind Singh in 1708.
Thereafter it refers to the Sikh Holy Scriptures the Guru Granth Sahib. The divine spirit was passed from one Guru to the next as "The light of a lamp which lights another does not abate. Similarly a spiritual leader and his disciple become equal, Nanak says the truth."