The Cantabrian labarum (Cantabrian: lábaru cántabru or Spanish: lábaro cántabro) is a modern interpretation of the ancient military standard known by the Romans as Cantabrum. It consists of a purple cloth on which there is what would be called in heraldry a "saltire voided" made up of curved lines, with knobs at the end of each line.

The name and design of the flag is in the theory advocated by several authors of a relationship between the genesis of labarum and the military standard called Cantabrum, thereby identifying both as a same thing; and the alleged relationship the Codex Theodosianus established between the Labarum and the Cantabrarii, the school of Roman soldiers in charge of carrying the Cantabrum.

Additionally, and according to the definition of the Royal Academy of the Spanish language, labarum is the Roman standard (as in military ceremonial flag) on which, under Emperor Constantine's rule, the cross and the Monogram of Christ (Chi-Rho) was drawn. By association of ideas, labarum can refer just to the monogram itself, or even just the cross.

Etymologically, the word comes from (p)lab- which means to speak in a number of Celtic languages, many of which have derivatives. For example, in Welsh llafar means "speech", "language", "voice". Ancient Cornish and Breton have lavar, "word", and ancient Irish has labrad: "language", "speech".