1. A cartilaginous rod that runs along the back (dorsal) of all chordates () at some point in their life cycle.
2. A rod consisting of cartilage running underneath, and supporting, the nerve cord.
3. In embryology, a rod-shaped body of cells that is the center of development of the axis of the skeleton.
4. A flexible rodlike structure that forms the main support of the body in all chordates (animals having a notochord, or dorsal stiffening rod) during some stage of their development.

In vertebrates, the notochord develops into a true backbone in the embryonic phase while primitive chordates; such as, lancelets and tunicates, keep a notochord throughout their existence.