1. A pointed spiny part of a structure: The sharp projection of a vertebra is termed the acantha.
2. The spine: The vertebrae in a person is called the acantha because of the many pointed segments.
3. In biology, a spike-like structure or spiny protrusion on a plant or animal: A wild rosebush has many acanthas, or thorns, that are quite similar to needles.

Bill read in his book about acanthas on insects being sharp projections, normally as a one-celled epidermal growth and lacking cells for the senses.