1. A flexible rodlike structure that forms the main support of the body in the lowest chordates; a primitive spine which are also found in the embryos of vertebrates from which the spine develops.
2. A phylum of the animal kingdom consisting of all of the animals that have a notochord (the primitive axis of the body) during some stage of their development.

It includes the subphyla cephalochordata, urochordata, and vertebrata (vertebrates).

3. A phylum of animals having a notochord, or dorsal stiffening rod, as the primary internal skeletal support at some stage of their development.

Most chordates are vertebrates (animals with backbones), but the phylum also includes some small marine invertebrate animals.

The three features unique to chordates and found in all of them at least during early development are:

• The notochord, composed of gelatinous tissue and bound by a tough membrane.
• A tubular nerve cord (or spinal cord), located above the notochord.
• Gill slits leading into the pharynx, or anterior part of the digestive tract (the throat, in higher vertebrates).

4. Etymology: Modern Latin, from Latin chorda, "cord"; from Old French corde, "rope, string, twist, cord"; from Latin chorda, "string of a musical instrument, cat-gut", from Greek, khorde, "string, catgut, chord, cord" + ending from Vertebrata which came from vertebra, which refers to "any of the bones or segments of the spinal column from Latin, "joint of the spine, from vertere, "to turn".