A serif that meets the main stroke at an abrupt angle rather than flowing smoothly out of it, characteristic of Neoclassical and Romantic typefaces.
Definition
A transitive serif is one that joins the main stroke at an abrupt, sharply defined angle — as if the pen were lifted and repositioned. This crisp, angular junction contrasts with the smooth, continuous curve of a reflexive serif. Transitive serifs are characteristic of Neoclassical and Romantic typefaces such as Bodoni and Didot, where the contrast between thick and thin strokes is pronounced and the serifs are thin, flat, and mechanically precise. The transitive serif reflects the rationalist approach to letter construction, prioritizing geometric clarity over calligraphic continuity.
Source
No access — Robert Bringhurst — p. 346:
A serif that meets the main stroke at an abrupt angle, as in a rationalist letter.