noun

• (historical) A round dance accompanied by singing.
  "The carol, a combination of dance, music and song performed by a group, has a parallel history [to the mystery plays]. Although it existed earlier as a secular form – the round dance of which St Hugh's biographer was reminded by the shafts at Lincoln – it seems to have been turned to pious uses from about 1350."
• A ballad or song of joy.
  "They sang a Christmas carol."
• (architecture, obsolete) A small closet or enclosure built against the inner side of a window of a monastery's cloister, to sit in for study.
  "Carol, or Carrel. A little pew, or closet, in a cloister, to sit and read in. They were common in greater monasteries, as Duram, Gloucester, Kirkham in Yorkshire, &c.; and had their name from the carols, or sentences inscribed on the walls about them, which often were couplets in rhyme. [Carola, Low Latin.]"

verb

• (intransitive, historical) To participate in a carol (a round dance accompanied by singing).
  "You might see the townsmen and the ladies carolling in the squares, squires and serjans and young girls singing; there is no street and no house to be found there that is not adorned with hangings of gold and silk."
• (intransitive) To sing in a joyful manner.
  "And when the ſtubborne ſtroke of ſtronger ſtounds, / Has ſomewhat ſlackt the tenor of thy ſtring; / Of loue and luſtihead tho maiſt thou ſing, / And carroll lowde, and leade the Millers rounde, [...]"
• (intransitive) To sing carols; especially to sing Christmas carols in a group.
  "Christmas morning was welcomed by services in some churches. Everyone in the Bingham house, along with other church members, went carolling at five o'clock in the morning, which culminated in the Christmas message at the church, delivered by the pastor. Everyone's heart was blessed."
• (transitive) To praise or celebrate in song.
  "[S]till ſhe [Sabrina, a water nymph] retaines / Her maiden gentleneſſe, and oft at eve / Viſits the heards along the twilight meadows, / Helping all urchin blaſts, and ill lucke ſignes, / That the ſhrewd medling elfe delights to make, / Which ſhe with precious viol'd liquors heales; / For which the ſhepheards at their feſtivalls / Carroll her goodneſſe lowd in ruſticke layes, / And throw ſweet garland wreaths into her ſtreame / Of pancies, pinks, and gaudie daffadills."