The Roman emperor Constantine I created a new military standard for his army which displayed the first two Greek letters of the name of Christ - Chi (χ) and Rho, (ρ) which came to be known as the labarum. The etymology of the word before Constantine's usage of it is unclear. According to Lactantius (On the Deaths of Persecutors, chapter 44), Constantine had dreamed of this emblem and a voice saying "In this sign you will conquer" (Latin: In hoc signo vinces). On waking he ordered his soldiers to put the emblem on their shields; that day they fought and won the Battle of the Milvian Bridge against the forces of Maxentius.
The labarum is sometimes referred to as the 'monogram' of Christ, though the word 'monogram' means 'single letter'.