The labarum (Greek: λάβαρον or λάβουρον) was a vexillum (military standard) that displayed the "Chi-Rho" symbol ☧, a christogram formed from the first two Greek letters of the word "Christ" (Greek: ΧΡΙΣΤΟΣ, or Χριστός) – Chi (χ) and Rho (ρ). It was first used by the Roman emperor Constantine the Great.

Ancient sources draw an unambiguous distinction between the two terms "labarum" and "Chi-Rho", even though later usage sometimes regards the two as synonyms. The name labarum was applied both to the original standard used by Constantine the Great and to the many standards produced in imitation of it in the Late Antique world, and subsequently.

Beyond its derivation from Latin labarum, the etymology of the word is unclear. The Oxford English Dictionary offers no further derivation from within Latin. Some derive it from Latin labare 'to totter, to waver' (in the sense of the "waving" of a flag in the breeze) or laureum [vexillum] ("laurel standard"). An origin as a loan into Latin from a Celtic language or Basque has also been postulated. There is a traditional Basque symbol called the lauburu; though the name is only attested from the 19th century onwards the motif occurs in engravings dating as early as the 2nd century AD.

Harry Thurston Peck, in his Harpers Dictionary of Classical Antiquities, wrote: "The etymology of the term itself has given rise to many conflicting opinions. Some derive the name from labor; others from εὐλάβεια, 'reverence'; others from λαμβάνειν, 'to take'; and others, again, from λάφυρα, 'spoils'. One writer makes Labarum to be like S.P.Q.R., only a notatio, or combination of initials to represent an equal number of terms; and thus, L.A.B.A.R.V.M. will stand for Legionum aquila Byzantium antiqua Roma urbe mutavit."

On the evening of October 27, 312 AD, with his army preparing for the Battle of the Milvian Bridge, the emperor Constantine I claimed to have had a vision which led him to believe he was fighting under the protection of the Christian God.

Lactantius states that in the night before the battle Constantine was commanded in a dream to "delineate the heavenly sign on the shields of his soldiers". Obeying this command, "he marked on their shields the letter X, with a perpendicular line drawn through it and turned round thus at the top, being the cipher of Christ". Having had their shields marked in this fashion, Constantine's troops readied themselves for battle.

From Eusebius, two accounts of a battle survive. The first, shorter one in the Ecclesiastical History leaves no doubt that God helped Constantine but does not mention any vision. In his later Life of Constantine, Eusebius gives a detailed account of a vision and stresses that he had heard the story from the emperor himself. According to this version, Constantine with his army was marching somewhere (Eusebius does not specify the actual location of the event, but it clearly is not in the camp at Rome) when he looked up to the sun and saw a cross of light above it, and with it the Greek words Ἐν Τούτῳ Νίκα. The traditionally employed Latin translation of the Greek is in hoc signo vinces— literally "In this sign, you will conquer." However, a direct translation from the original Greek text of Eusebius into English gives the phrase "By this, conquer!"

At first he was unsure of the meaning of the apparition, but the following night he had a dream in which Christ explained to him that he should use the sign against his enemies. Eusebius then continues to describe the labarum, the military standard used by Constantine in his later wars against Licinius, showing the Chi Rho sign.