Dalmatic. From the Latin dalmatic (= coming from Dalmatia, Balkan region), the Dalmatic is a tunic with short and very wide sleeves and a distinctive ornamentation of two vertical stripes. Imported from Dalmatia during the second century, this prestigious garment soon became one of the robes of the imperial court aristocracy.
“The vestment proper to the deacon is the dalmatic, worn over the alb and stole. The dalmatic may, however, be omitted out of necessity or on account of a lesser degree of solemnity.” (GIRM no. 338.).
Like the chasuble it is supposed to match the liturgical colour of the day.