Intercalary

1. Describing a time period inserted between others; leap, (as in leap day, leap month, or leap year)
2. (sciences, by extension) Inserted between other things
   1. (botany) 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.
   2. (entomology) of a wing vein: between the major veins common to insect wings.