Too complicated to explain here. Go here: http://en.wikipedia.org/wiki/Id,_ego_and_super-ego

ID is the abbreviation for Identification and it is used everywhere in both general and formal English. EGO and SUPER-EGO are not the abbreviations. These are comlpete words themselves.

ID is abbreviation of identification. But EGO is really.. :)

I thought it was the right answer, sorry. Eliot told me before. In Spanish is 'ELLO', and the word itself doesn't have a meaning.

The "best answer" is incorrect! id in the sense of id, ego, and superego is not an acronym, each letter does not have a meaning. id in the sense of id, ego, and superego is pronounced like kid, hid, lid, and bid. ID in the sense of identification is pronounced "eye dee", spelling the letters. They are two different words with completely different meanings!