hypocoristic

http://www.randomhouse.com/wotd/index.pperl?date=19990714

The word hypocoristic is not uncommon in linguistics. As an adjective, it means 'endearing, as a pet name or a diminutive'; as a noun, 'a hypocoristic form; a pet name or a diminutive'. The adjective is much more common.

 Hypocoristic forms are, as the definition says, generally either pet names, or nicknames, such as Harry for Henry or Betsy or Beth or Liz or about a zillion others for Elizabeth, or they are forms with some sort of diminutive element, such as the suffix -y/-ie, yielding such words as preppy (prep (school) + -y), kiddie, birdie, cutie, and the like.