Entry 1 sense Webster, 1913 Midrib /mĭd'-rĭb/ · Mid·rib · IPA /ˈmɪdrɪb/ 01 n. A continuation of the petiole, extending from the base to the apex of the lamina of a leaf. 1. A continuation of the petiole, extending from the base to the apex of the lamina of a leaf.(Bot.)