Apophyge

An apophyge (Greek Ancient Greek: ἀποφυγή, a flying off), in architecture, is the lowest part of the shaft of an Ionic or Corinthian column, or the highest member of its base if the column be considered as a whole.

The apophyge is the inverted cavetto or concave sweep, on the upper edge of which the diminishing shaft rests.

[1] It is, in effect, a curved join between the column shaft and the slightly larger section of the column to which the shaft joins.

It bridges the two radii to avoid a sharp change.

This was likely done to avoid cracking of the marble in ancient structures.