An invisible proto-language is a ad hoc technical term used to refer to a constructed proto-language that is invisible to anyone other than the creator. It's a tool to be used by a conlanger to create another language, but is rarely showcased as a language on its own. This contrasts with a visible proto-language, which is itself a full conlang that is intended for use. For example, the proto-language of Astapori Valyrian is High Valyrian. High Valyrian is a visible proto-language, in that it is also presented as a full conlang. The proto-language of High Valyrian, on the other hand, is Old Valyrian, which is an invisible proto-language. It's not fleshed out to the same extent, and is not intended to serve as a fully functioning conlang.
Given the fact that we still do not know how language emerged for the very first time, an invisible proto-language is likely a necessarily evil in naturalistic conlanging. Much the way one can posit proto-forms for Indo-European languages going all the way back to Proto-Indo-European but no further, a conlanger must state that at a certain point one can delve no further into the history of their conlang.
As a general note, for this wiki, only a priori languages will be included in this list. A language like Lishepus, whose proto-language is essentially a mix of different natural language roots with Proto-Semitic grammar, will not be listed here.