I had to look it up too, Anna, although I have the feeling it's been used here before. (Just Searched--yep, it has; Dr. Bill's given the def. at least twice!) Gurunet gives:

hy·poc·o·rism (hī-pŏk'ə-rĭz'əm, hī'pə-kôr'ĭz'əm, -kōr'-)
n.

1. A name of endearment; a pet name.
2. The use of such names.
[Late Latin hypocorisma, from Greek hupokorisma, from hupokorizesthai, to call by endearing names : hypo-, beneath, secretly; see hypo– + korizesthai, to caress (from koros, boyor korē, girl).]

The etymology given made me wonder if charisma is related.