The distinctive vestment of deacons in the western church. It may be worn at any liturgy in any season. The term is derived from a white tunic worn in second-century Dalmatia. The dalmatic was an ample white tunic with wide sleeves, bands about the cuffs, and clavi, or colored bands, descending from the shoulders to the hem. Historically, it was worn over an alb by both bishops and deacons by the fourth century, but it did not become a vestment until around the ninth century. The dalmatic was accepted as the vestment worn by the deacon at the eucharist by the ninth century. Eventually deacons adopted the eastern orarion or stole, worn on top and hanging straight down from the left shoulder. Over the centuries the dalmatic, like other vestments, lost its full shape. The stole disappeared beneath the outer garment. By the late middle ages, deacons (or, more commonly, priests acting as liturgical deacons) were wearing a short dalmatic in the color of the day, ornate in fabric, adorned with orphreys (two vertical and two horizontal), with narrow sleeves, and open at the sides. The dalmatic has varied widely in appearance, and this variety continues in the Episcopal Church today. In many places the medieval dalmatic has given way to a full-length white or off-white tunic which is simple and functional. It is worn sometimes over an alb, sometimes by itself (as a combination cassock-alb-dalmatic). Deacons often wear the stole on top, placed over the left shoulder and tied under the right arm or hanging straight down.
From Don S. Armentrout’s An Episcopal Dictionary of the Church: A User-Friendly Reference for Episcopalians Church Publishing Inc., 2000.