(
Etymology)
So-called asterisk (*) words are hypothetical ancestors of given words in languages. They have not been found to be attested in any written source, but have been reconstructed by a process of analysis and deduction. For example, the Indo-European *ped-, the root of words for "foot" in most of its daughter tongues, has been reconstructed in this way.
As their name suggests, asterisk-words are denoted by an asterisk '*' tagged on the front.
N.B. This form should not be confused with the "double asterisk" '**' which denotes incorrect, non-existent forms. For example, to quote Irene Gates' critique of Ruth S. Noel's The Language's of Tolkien's Middle Earth:
"**cheb 'keep' cannot be a Sindarin word, as there is no initial ch in Sindarin. It is the lenited form of either *ceb or *heb."
- from http://www.elvish.org/articles/LRH.html
You learn something new every day!