The word སྤྲུལ or 'sprul' (Modern Lhasa Tibetan ) was a verb in Old Tibetan literature and was used to describe the བཙན་པོ་ btsanpo ('emperor'/天子) taking a human form on earth. So the "sprul" idea of taking a corporeal form is a local religious idea alien to Indian Buddhism and other forms of Buddhism (e.g. Theravadin or Zen). Over time, indigenous religious ideas became assimilated by the new Buddhism; e.g. "sprul" became part of a compound noun, སྤྲུལ་སྐུ་'sprul.sku' ("incarnation body" or 'tülku', and 'btsan', the term for the imperial ruler of the Tibetan Empire, became a kind of mountain deity). The term "tülku" became associated with the translation of the Sanskrit philosophical term "nirmanakaya". According to the philosophical system of "trikaya" or "three bodies of Buddha", nirmanakaya is the Buddha's "body" in the sense of the bodymind (Sanskrit: "nāmarūpa"). Thus, the person of Siddhartha Gautama, the historical Buddha, is an example of nirmanakaya. In the context of Tibetan Buddhism, "tülku" means the corporeal existence of enlightened Buddhist masters in general.