yi-dam (Tibetan). A tutelary deity associated with tantric Buddhism. The image and attributes of the deity are appropriated and used during personal meditation (sādhana) for the purpose of transformation through self-identification. Examples of yi-dams include Cakra-saṃvara, Hayagrīva, Heruka, Mahākāla, Vajrabhairava, and Vajrayoginī, all of whom have a distinctive iconography and are the object of cultic practices such as rites of propitiation.

A Dictionary of Buddhism, Oxford University Press, 2003, 2004 (which is available in electronic version from answer.com)