Asterisk
The asterisk is a symbol used in historical linguistics. Tolkien made use of asterisks in his texts about Elvish languages.
Usage
In historical linguistics, an asterisk immediately before a word indicates that the word is not directly attested, but has been reconstructed on the basis of other linguistic material (see also comparative method). The asterisk shows that this form of word is hypothetical and historically it could be more or less different.
Asterisks are therefore used mostly in the reconstruction unrwritten or prehistorical languages, deduced by comparison between other languages.
For example the Old English word endleofan suggests a Proto-Germanic form was *ainlif. Proto-Germans did not have a writing, therefore ainlif is unattested, and this is indicated with an asterisk.
Elvish
Tolkien used asterisks when writing about the history of his Elvish languages, to show that for some reason, these forms are "unattested", and belong in a stage of a language before the Tengwar or the Cirth.