The Lombard language (Lombard: lombard, lumbard, lumbart or lombart, depending on the orthography; pronunciation: [lũˈbaːrt, lomˈbart]) belongs to the Gallo-Italic group within the Romance languages. It is characterized by a Celtic linguistic substratum and a Lombardic linguistic superstratum and is a cluster of homogeneous dialects that are spoken by millions of speakers in Northern Italy and southern Switzerland. These include most of Lombardy and some areas of the neighbouring regions, notably the far eastern side of Piedmont and the extreme western side of Trentino, and in Switzerland in the cantons of Ticino and Graubünden. The language is also spoken in Santa Catarina in Brazil by Lombard immigrants from the Province of Bergamo, in Italy.

The most ancient linguistic substratum that has left a mark on the Lombard language is that of the ancient Ligures. However, available information about the ancient language and its influence on modern Lombard is extremely vague and limited. That is in sharp contrast to the influence left by the Celts, who settled in Northern Italy and brought their Celtic languages and culturally and linguistically Celticised the Ligures. The Celtic substratum of modern Lombard and the neighbouring languages of Northern Italy is self-evident and so the Lombard language is classified as a Gallo-Italic language (from the ancient Roman name for the Celts, Gauls).

Roman domination shaped the dialects spoken in the area, which is called Cisalpine Gaul ("Gaul, this side of the mountains") by the Romans, and much of the lexicon and grammar of the Lombard language have their origin in Latin. However, that influence was not homogeneous since idioms of different areas were influenced by previous linguistic substrata, and each area was marked by a stronger or weaker Latinisation or the preservation of ancient Celtic characteristics.

The Germanic Lombardic language also left strong traces in modern Lombard, as it was the variety of Germanic that was spoken by the Germanic Lombards (or Longobards), who settled in Northern Italy, which is called Greater Lombardy after them, and in other parts of the Italian Peninsula after the fall of the Western Roman Empire. Lombardic acted as a linguistic superstratum on Lombard and neighboring Gallo-Italic languages since the Germanic Lombards did not impose their language by law on the Gallo-Roman population, but they rather acquired the Gallo-Italic language from the local population. Lombardic left traces, mostly in lexicon and phonetics, without Germanicising the local language in its structure and so Lombard preserved its Romance structure.

From the 15th century onwards, literary Tuscan began to supplant the use of northern vernaculars such as Lombard, even regardless of the fact that Lombard itself began to be heavily influenced by the Tuscan vernacular. Prior to that, the Lombard language was widely used in administrative spheres. Among those who favoured the strengthening of Tuscan influences over Lombard culture was the Duke of Milan Ludovico il Moro; during his reign he brought numerous men of culture from the Republic of Florence to the Sforza court, the most famous of whom was certainly Leonardo da Vinci. At the same time, however, Lancino Curzio still wrote some works in Milanese dialect at the Sforza court.

Between the 15th and 16th centuries, the Lombard language was widely and actively discredited in Italian literary circles. Tuscan writers and humanists such as Luigi Pulci and Benedetto Dei recorded aspects of the language spoken in Milan in the form of parodies; similarly, the Asti-born writer Giorgio Alione parodied Milanese in his Commedia e farse carnovalesche nei dialetti astigiano, milanese e francese misti con latino barbaro (eng. "Comedy and carnival farces in the Asti, Milanese and French dialects mixed with barbaric Latin") composed at the end of the 15th century. The Florentine humanist Leonardo Salviati, one of the founders of the Accademia della Crusca, an important Italian linguistic academy operating to this day, published a series of translations of a Boccaccian tale into various vernaculars (including Bergamo and Milanese) explicitly in order to demonstrate how ugly and awkward they were compared to Tuscan.

At the same time, the 15th century saw the first signs of a true Lombard literature: in the eastern parts of Lombardy, the Bergamo-born Giovanni Bressani composed numerous volumes of satirical poetry and the Brescia-born Galeazzo dagli Orzi wrote his Massera da bé, a sort of theatrical dialogue; in the west of the region area, the Mannerist painter Giovanni Paolo Lomazzo lead the composition of the "arabesques" in the Accademia dei Facchini della Val di Blenio, a Milanese academy founded in 1560.

At the beginning of the 17th century, the Ossola native Giovanni Capis published the Varon milanes de la lengua de Milan (eng. "Varrone Milanese on the language of Milan"), a sort of etymological dictionary was published.