intercalary

Short definition from Wiktionary

Adjective

1. 1
   Describing a time period inserted between others; leap, (as in leap day, leap month, or leap year)
2. 2
   Inserted between other things
3. 3
   Inserted between other things: of a meristem: situated between zones of permanent tissue, thus a shoot growing at the base of a leaf, in comparison with apical growth at the tip of a root or plant.
4. 4
   Inserted between other things: of a wing vein: between the major veins common to insect wings.

Noun

1. 1
   Such a time period