Is Id a common noun or proper noun?

Answer 1

The noun 'id' (the part of your mind that is unconscious that has hidden wants and needs) is a common noun, a word for anyone's id. The noun id is only capitalized when it is the first word in a sentence.

The word 'ID' is an abbreviation for 'identification' (a document that gives the details of your name, address, and date of birth, sometimes with a photograph) is also a common noun, a word for any ID of any kind. The abbreviation is capitalized to distinguish it from the word 'id'; this use of capitalization does not make ID a proper noun.