noun.
An internal supporting skeleton, derived from the mesoderm, that is characteristic of vertebrates and certain invertebrates.
The Century Dictionary and Cyclopedia
noun.
In anatomy, the internal skeleton or framework of the body; the whole bony, chitinous, cartilaginous, or other hard structure which lies within the integument, and is covered by flesh and skin, as distinguished from the exoskeleton.
the GNU version of the Collaborative International Dictionary of English
noun.
The bony, cartilaginous, or other internal framework of an animal, as distinguished from the exoskeleton.