Linguistic reconstruction

Linguistic reconstruction is the practice of establishing the features of an unattested ancestor language of one or more given languages. There are two kinds of reconstruction:

In texts concerning linguistic reconstruction, reconstructed forms are commonly prefaced with an asterisk (*), to distinguish them from attested forms.

An attested word from which a root in the proto-language is reconstructed is a reflex. More generally, a reflex is the known derivative of an earlier form, which may be either attested or reconstructed. Reflexes of the same source are cognates.

...
Wikipedia