The Collaborative International Dictionary of English v.0.48:

Intercalation \In*ter`ca*la"tion\, n. [L. intercalatio: cf. F.
   intercalation.]
   [1913 Webster]
   1. (Chron.) The insertion of a day, or other portion of time,
      in a calendar.
      [1913 Webster]

   2. The insertion or introduction of anything among others, as
      the insertion of a phrase, line, or verse in a metrical
      composition; specif. (Geol.), the intrusion of a bed or
      layer between other layers.
      [1913 Webster]

            Intercalations of fresh-water species in some
            localities.                           --Mantell.
      [1913 Webster]

WordNet (r) 3.0 (2006):

intercalation
    n 1: an insertion into a calendar [syn: embolism,
         intercalation]