Definition
A serif that flows in one direction from the stem, bending back toward the direction from which it came. Reflexive serifs are found, for example, on the ascender of d or the descender of p in many typefaces. They contrast with transitive serifs, which flow through the stem and out the other side.
Source
— Robert Bringhurst — p. 329:
A serif that flows in one direction from the stem, bending back toward the direction from which it came.