A dalmatic is the outer liturgical vestment of the deacon worn at Mass, solemn processions, and benedictions. It is similar in shape and function to a chasuble, but has sleeves, allowing the deacon to assist preparations for the Mass more easily. A dalmatic is considered a festive vestment; therefore, it is not worn during somber periods like Advent or Lent. Bishops also wear a dalmatic under their chasuble at pontifical Mass, but not private Mass. Priests may only wear a dalmatic if they have been granted a special papal privilege and then only on the days for which they have received permission.

Sources:

Braun, Joseph. "Dalmatic." In The Catholic Encyclopedia. Robert Appleton Company, 1908. http://www.newadvent.org/cathen/04608a.htm.

The History of Liturgical Vestments in the Catholic Church. The San Felipe de Neri Parish Museum, 2022. https://sanfelipedeneri.org/documents/2022/1/The%20History%20of%20Liturgical%20Vestments%20in%20the%20Catholic%20Church-1.pdf.