adjective
(medicine, of a fever) Characterised by paroxysms recurring every other day (that is, every third day by inclusive reckoning).
(music) Pertaining to the mean-tone temperament, in which major thirds are perfectly in tune.
noun
A tertian fever.
A cask of a certain size; its volume used as a measure of capacity (one-third of a tun's).
A Jesuit going through tertianship.