in·ter·ca·la·ry /ɪnˈtɝkəˌlɛri, ˌɪntɚ/
  (a.)閏的,插入中間的,被夾于中間的
  In·ter·ca·la·ry a.
  1. Chron. Inserted or introduced among others in the calendar; as, an intercalary month, day, etc.; -- now applied particularly to the odd day (Feb. 29) inserted in the calendar of leap year. See Bissextile, n.
  2. Introduced or inserted among others; additional; supernumerary. “Intercalary spines.”
     This intercalary line . . . is made the last of a triplet.   --Beattie.
  Intercalary day Med., one on which no paroxysm of an intermittent disease occurs.
  ◄ ►
  intercalary
       adj : added or having an addition to make the calender year
             correspond to the solar year; "Feb. 29 is an
             intercalary day"; "leap year is an intercalary year"