INTERCALATION

intercalation. Intercalation, n. the insertion of an odd day

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In‐ter′ca‐la″tion (?), n. [[L. intercalatio: cf. F. intercalation.]] 1. 1. (Chron.) The insertion of a day, or other portion of time, in a calendar.
2. 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.
Intercalations of fresh-water species in some localities. Mantell.