Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- A group of related, similar species recognized by taxonomists with a distinct name used in binomial nomenclature.
Glossary of "A Botanist's Vocabulary" by Pell & Angell (2016)
- Sadava, D., Hillis, D. M., Heller, H. C. & Berenbaum, M. R. 2011. Life: The Science of Biology, 9th ed. -xliv + 1266 pp. Sinauer Associates, Inc., Sunderland.
- Pell, S. K. & Angell, B. 2016. A Botanist's Vocabulary: 1300 Terms Explained and Illustrated. 226 pp. Timber Press, Portland.