A hypothetical lost parent language from which actual languages are derived is called

The hypothetical lost parent language from which real languages descend is called a protolanguage.

Scholars reconstruct it by comparing related descendant tongues, as with Proto Indo European.

Para language refers to vocal features like pitch, tone and pauses, not an ancestral tongue.

The label derivative language would apply to one that comes from another, the opposite of an original ancestor.

Metalanguage is language used to describe language itself, a tool of analysis rather than a lost root.

The prefix proto signals first or earliest, marking the ancestral source form.