Textual critics should only use the word archetype to designate the point in the stemma beyond which the surviving tradition does not allow them to reach. In the light of the above considerations, the most complete and rigorous definitions of archetype offered by the recent manuals I am familiar with are those of Blecua and Stussi. These authors also explain the logico-formal conditions proving the existence of an archetype:

Un códice o impreso perdido, X, o conservado, A, B, C, etc., que transmita errores communes a todos los testimonios (Blecua, 71).

A lost copy marred by at least one error of the conjunctive type, from which the whole tradition derives (Stussi, 121).

However, although it may seem superfluous […], they should possibly be reformulated as follow:

Un códice o impreso, perdido […], que transmita errores communes a todos los testimonios conservados.

A lost copy marred by at least one error of the conjunctive type, from which the whole surviving tradition derives.