A chordate ( KOR-dayt) is a bilaterian animal belonging to the phylum Chordata ( kor-DAY-tə). All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa. These five synapomorphies are a notochord, a hollow dorsal nerve cord, an endostyle or thyroid, pharyngeal slits, and a post-anal tail.

In addition to the morphological characteristics used to define chordates, analysis of genome sequences has identified two conserved signature indels (CSIs) in their proteins: cyclophilin-like protein and inner mitochondrial membrane protease ATP23, which are exclusively shared by all vertebrates, tunicates and cephalochordates. These CSIs provide molecular means to reliably distinguish chordates from all other animals.

Chordates are divided into three subphyla: Vertebrata (fish, amphibians, reptiles, birds and mammals), whose notochords are replaced by a cartilaginous/bony axial endoskeleton (spine) and are cladistically and phylogenetically a subgroup of the clade Craniata (i.e. chordates with a skull); Tunicata or Urochordata (sea squirts, salps, and larvaceans), which only retain the synapomorphies during their larval stage; and Cephalochordata (lancelets), which resemble jawless fish but have no gills or a distinct head. The vertebrates and tunicates compose the clade Olfactores, which is sister to Cephalochordata (see diagram under Phylogeny). Extinct taxa such as the conodonts are chordates, but their internal placement is less certain. Hemichordata (which includes the acorn worms) was previously considered a fourth chordate subphylum, but now is treated as a separate phylum which are now thought to be closer to the echinoderms, and together they form the clade Ambulacraria, the sister phylum of the chordates. Chordata, Ambulacraria, and possibly Xenacoelomorpha are believed to form the superphylum Deuterostomia, although this called into doubt in a 2021 publication.

Chordata is the third-largest phylum of the animal kingdom (behind only the protostomal phyla Arthropoda and Mollusca) and is also one of the most ancient animal taxa. Chordate fossils have been found from as early as the Cambrian explosion over 539 million years ago. Of the more than 81,000 living species of chordates, about half are ray-finned fishes (class Actinopterygii) and the vast majority of the rest are tetrapods, a terrestrial clade of lobe-finned fishes (Sarcopterygii) who evolved air-breathing using lungs.

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