Nomenclature and etymology
The word སྤྲུལ or 'sprul' (Modern Lhasa Tibetan [ʈʉl]) 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 is used to refer to the corporeal existence of enlightened Buddhist masters in general.
In addition to Tibetans and related peoples, Tibetan Buddhism is a traditional religion of the Mongols and their relatives. The Mongolian word for a tülku is qubilγan, though such persons may also be called by the honorific title qutuγtu (Tib: 'phags-pa and Skt: ārya or superior, not to be confused with the historic figure, 'Phags-pa Lama or the script attributed to him, (Phags-pa script), or hutagt in the standard Khalkha dialect. According to the Light of Fearless Indestructible Wisdom by Khenpo Tsewang Dongyal: the term tülku "designates one who is 'noble' (or 'selfless' according to Buddha's usage) and used in Buddhist texts to denote a highly achieved being who has attained the first bhumi, a level of attainment which is truly egoless, or higher."
The Chinese word for tülku is huófó (活佛), which literally means "living Buddha" and is sometimes used to mean tülku, although the Dalai Lama has said that this is a mistranslation, as a tülku isn't necessarily a realized being.