- 1: An embryonic movement that involves streaming of material from the dorsal and lateral surfaces of the gastrula toward the blastopore and concurrent shifting of lateral materials toward the middorsal line and that is a process fundamental to the establishment of the germ layers.
2: Independent development of similar characters (as of body structure in whales and fishes) by animals or plants of different groups that is often associated with similarity of habits or environment.
3: Movement of the two eyes so coordinated that the images of a single point fall on corresponding points of the two retinas.
4: Overlapping synaptic innervation of a single cell by more than one nerve fiber.
Definition from: Merriam-Webster's Medical Dictionary by Merriam-Webster Inc.
See also Understanding Medical Terminology.