What did Romans call their language?

Question

I was taught that Latinus is an adjective related to the area of Latium. Latin would be called lingua Latina, "the language of Latium", never merely Latina. There is a single-word expression referring to the language, Latinitas, but it has a different connotation than lingua Latina.

What did ancient Romans really call their language? Was it ever just Latina (which I was emphatically taught to be wrong), or always lingua Latina (possibly with lingua replaced with another appropriate word like sermo) or Latinitas? I have only found examples of lingua Latina, so I would be curious to see if any other alternatives were used.

Answer 1

All the terms you used are used by Classical authors (and then some), but some differences must be noted.

Lingua Latina is what the Romans called their language. If you ever see Latina by itself to refer to language, lingua is what is implied. However, that typically wasn't the way they referred to speaking the language. Instead, the adverbial form was preferred: Latine loqui, "to speak Latin". The earliest attestation goes all the way back to Plautus. In the Poenulus (1029), Hanno, a Phoenician character say he will speak in Latin:

"At ut scias, nunc dehinc latine iam loquar."

This is done not just with Latin, but all sorts of languages.

"Acilius qui Graece scripsit historiam."
"Acilius who wrote a history in Greek." (Cic. Off. 2.32.115; cf. graece loqui Cic. Tusc. 1.8.15)

We also have sermo latinus, which literally means "the Latin speech," as opposed to litterae, which are written down, but this distinction fails to be followed in ordinary, regular usage:

quae philosophi Graeco sermone tractavissent, ea Latinis litteris mandaremus.
I commit these things [to writing] with Latin letters what the philosophers have [already] handled in Greek speech. (Cic. Fin. 1.1.1.)

Clearly Graeco sermone is meant Greek language both written and spoken―Cicero isn't listening to the old Greek philosophers, but reading them.

Latinitas is something else entirely. This does not refer to the language, but the art of speaking and writing the language. Whoever does not make grammatical mistakes, whose command of the language is good, that person embodies Latinitas. As the author of the Rhetorica ad Herennium 4.12.17 notes:

latinitas est, quae sermonem purum conservat, ab omni vitio remotum.

Latinitas is that which conserves pure speech, free of all vice.

Answer 2

The most common form of expressing "Latin" when referring to the language is, as you note, to use lingua latina. Here are a few examples, many taken from the Lewis & Short entry for lingua:

ita sentio et saepe disserui, Latinam linguam non modo non inopem, ut vulgo putarent, sed locupletiorem etiam esse quam Graecam. (Cicero, de Finibus Bonorum et Malorum, 1, 3)

I feel and often have stated that the Latin Linguage not only is not poor, as it is often popularly reputed, but also is even richer than Greek.

or

Quod quidem Latina lingua sic observat... (Cicero, Orator, 44, 150)

Cicero also provides us with many examples of sermo Latinus, e.g.:

cui (Catulo) non solum nos Latini sermonis, sed etiam Graeci ipsi solent suae linguae subtilitatem elegantiamque concedere (Cicero, de Oratore, 2, 7, 28)

Not only do we accept [Catullus] as a subtle and elegant master of the Latin language, but even the Greeks often accept him as such for their own language.

and

in Latino sermone (Cicero, de Oratore, 3, 11, 42)

Another common word used to mean in Latin--as is often meant when we say Latin in English--is the adverb latine. Thus, to speak Latin is often rendered as latine loqui.

Cicero's de Finibus Bonorum et Malorum contains several uses of the adjective latinus, -a, -um referring to the language alone. This excerpt contains three uses of "Latinus" as well as (for kicks) a more colloquial way of referring to Latin as the "mother tongue" or "native language":

Iis igitur est difficilius satis facere, qui se Latina scripta dicunt contemnere. in quibus hoc primum est in quo admirer, cur in gravissimis rebus non delectet eos sermo patrius, cum idem fabellas Latinas ad verbum e Graecis expressas non inviti legant. quis enim tam inimicus paene nomini Romano est, qui Ennii Medeam aut Antiopam Pacuvii spernat aut reiciat, quod se isdem Euripidis fabulis delectari dicat, Latinas litteras oderit? (Cic. Fin. 1, 2)

It is therefore rather difficult to satisfy those who claim to despise Latin writings. In such people I rather wonder, for one thing, why their native tongue does not please them in grave matters, when they are not unwilling to read Latin fables translated literally from Greek ones. For who is so hostile to the Roman name that he would despise and reject the Medea of Ennius or the Antiopa of Pacuvius while claiming to delight in the fables of Euripides?

(my rough translation)

As James noted in his answer, this adjective latinus can be used as a substantive as well, in the neuter.

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Latinitas is translated as "pure Latin style, Latinity" in its L&S entry and has a more precise usage that means much more than "Latin." See the above entry for examples of its use.

Answer 3

Here is one example of Cicero using it:

nam si quis minorem gloriae fructum putat ex Graecis versibus percipi quam ex Latinis, vehementer errat, propterea quod Graeca leguntur in omnibus fere gentibus, Latina suis finibus exiguis sane continentur

The linked dictionary entry of Latinus has this definition:

of Latium, Latin: genus, the Romans, V.: lingua (opp. Graeca): (fabulae), in Latin, T.: litterae, S.: nomen, Latin citizenship, C., S.: res, O.—As subst n.: in Latinum illa convertere, the Latin language.

There is then an example (from Cicero, no less) of Latinus the adjective being used in substantive form (Latina) to refer to the language we call Latin.