Isidore of Seville, Etymologies 1.32:
Barbarism is a word pronounced with a corrupted letter or sound. In the case of a letter, you might have floriet where it is appropriate to say florebit. In the case of sound, the first syllable might be drawn forth in place of the middle, as in latebrae, tenebrae. It is called barbarism after barbarians, since they were ignorant of the purity of Latin speech. For every people who became part of the Romans transmitted, along with its resources, all of its faults in language and customs.
But between barbarism and barbarolexis, there is this difference, that barbarism occurs in a Latin word when it is corrupted. When, however, barbarian words are brought into Latin speech, it is called barbarolexis. Similarly, when a fault of speech occurs in prose, it is called barbarism; but when it occurs in verse, it is called metaplasmus.
Barbarism, however, occurs both in writing and in pronunciation. In writing, it can take four forms: if someone adds, subtracts, transposes, or reduces a letter or syllable in a word. In pronunciation, it happens in time, tones, aspiration, and the rest which follow. In time, barbarism occurs if a short syllable is used in place of a long or a long in place of a short. In tone, it occurs if an accent is transferred to other syllables. In aspiration, it happens if H is added where it is not appropriate, or taken from a place where it belongs. In hiatus, it happens whenever a verse is broken off in pronunciation before it is completed, or whenever a vowel follows another vowel, as in the phrase ‘Musae Aonides.’
Barbarism can occur through motacismos, iotacismos, and labdacismos. Motacismos occurs whenever a vowel follows the letter M, as in ‘bonum aurum’, ‘iustum amicum’; but we can avoid this problem either by the suspension of the letter M or its removal. Iotacismus occurs whenever the sound of the letter i is duplicated, as in words like ‘Troia’ and ‘Maia’, where the pronunciation of those letters is so light than one iota, not two seem to be pronounced. Labdacismus is when two Ls are pronounced in place of two, as people from Africa tend to do, as in words like ‘colloquium’ instead of ‘conloquium’; or whenever we pronounce one L rather lightly, but two Ls more generously. Which is bass-ackwards. We should pronounce one L more expansively, while pronouncing Ls lighter when part of a pair. Collision occurs whenever the end of the last syllable is the beginning of the next syllable, as in the word ‘matertera’.
Barbarismus est verbum corrupta littera vel sono enuntiatum. Littera, ut “floriet” dum “florebit’ dicere oporteat; sono, si pro media syllaba prima producatur, ut “latebrae” “tenebrae” Appellatus autem barbarismus a barbaris gentibus, dum latinae orationis integritatem nescirent. Vnaquaeque enim gens facta Romanorum cum opibus suis vitia quoque et verborum et morum Romam transmisit. Inter barbarismum autem et barbarolexim hoc interest, quod barbarismus in verbo latino fit, dum corrumpitur; quando autem barbara verba latinis eloquiis inferuntur, barbarolexis dicitur. Item quando in prosa vitium fit sermonis, barbarismus vocatur; quando in metro, metaplasmus dicitur. Barbarismus autem fit scripto et pronuntiatione. Scripto quattuor modis: si quis in verbo litteram vel syllabam adiciat, mutet, transmutet, vel minuat. Pronuntiatione autem fit in temporibus, tonis, aspirationibus et reliquis quae sequuntur. Per tempora quippe fit barbarismus, si pro longa syllaba brevis ponatur, aut pro brevi longa. Per tonos, si accentus in alia syllaba commutetur. Per aspirationem, si adiciatur H littera ubi non debet, aut detrahatur ubi esse oportet. Per hiatum, quotiens in pronuntiatione scinditur versus antequam conpleatur, sive quotiens vocalis vocalem sequitur, ut “Musae Aonides” Fit barbarismus et per motacismos, [iotacismos] et labdacismos. Motacismus est, quotiens M litteram vocalis sequitur, ut “bonum aurum” “iustum amicum’; sed hoc vitium aut suspensione M litterae, aut detractione vitamus.Iotacismus est, quotiens in iota littera duplicatur sonus, ut “Troia” “Maia’; ubi earum litterarum adeo exilis erit pronuntiatio, ut unum iota, non duo sonare videantur. Labdacismus est, si pro una L duo pronuntientur, ut Afri faciunt, sicut “colloquium” pro “conloquium’; vel quotiens unam L exilius, duo largius proferimus. Quod contra est; nam unum largius, duo exilius proferre debemus. Conlisio est, quotiens novissimae syllabae finis in alterius principio est, ut “matertera”.