In linguistics, an asterisk often appears before a form (a word, sentence, etc.), where it has one of two functions:
  1. introducing a form that is ungrammatical or otherwise unacceptable, for the purposes of discussion;
  2. introducing a form that is posited but not directly attested, such as a reconstructed form.

It will be seen that there is a certain contradiction between these two usages: one means approximately "wrong" and the other something like "should be". The contradiction can be avoided by remembering that the asterisk actually indicates only that a form is unattested.