The labarum (Greek: λάβαρον) was a vexillum (military standard) that displayed the "Chi-Rho" symbol ☧, 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 I. Since the vexillum consisted of a flag suspended from the crossbar of a cross, it was ideally suited to symbolize the crucifixion of Christ.
The etymology of the word labarum is unclear; it is perhaps to be derived from Latin /labāre/ 'to totter, to waver' (in the sense of the "waving" of a flag in the breeze). Other proposals include a derivation from Celtic llafar ("eloquent"), from the Latin laureum ("laurel standard") or from ancient Cantabri dialect labaro ('four heads') (in modern-day Basque the word is lauburu, with the same meaning), an ancient Celtic symbol taken by the Legions during the Cantabrian Wars.
Later usage has sometimes regarded the terms "labarum" and "Chi-Rho" as synonyms. Ancient sources, however, draw an unambiguous distinction between the two.
Read more about Labarum: Vision of Constantine, Eusebius' Description of The Labarum, Iconographic Career Under Constantine, Later Usage, Modern Interpretations of Constantine's Vision