n. A taxonomic category below phylum and above class
The Century Dictionary and Cyclopedia
n. A prime division or main branch of a phylum; a group of a grade next below that of a phylum.
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n. (biology) a taxonomic group ranking between a phylum and a class
Word Usage
"Mammals are then grouped with the classes of other backboned animals, such as reptiles, into a "subphylum," Vertebrata, which is part of the "phylum" Chordata, containing all animals which have a nerve chord at some time in their life cycle."