The Collaborative International Dictionary of English v.0.48:

Theatin \The"a*tin\, Theatine \The"a*tine\, n. [F. th['e]atin,
   It. theatino.] (R. C. Ch.)
   1. One of an order of Italian monks, established in 1524,
      expressly to oppose Reformation, and to raise the tone of
      piety among Roman Catholics. They hold no property, nor do
      they beg, but depend on what Providence sends. Their chief
      employment is preaching and giving religious instruction.
      [1913 Webster]

   Note: Their name is derived from Theate, or Chieti, a city of
         Naples, the archbishop of which was a principal founder
         of the order; but they bore various names; as, Regular
         Clerks of the Community, Pauline Monks, Apostolic
         Clerks, and Regular Clerks of the Divine Providence.
         The order never flourished much out of Italy.
         [1913 Webster]

   2. (R. C. Ch.) One of an order of nuns founded by Ursula
      Benincasa, who died in 1618.
      [1913 Webster]