Glossary of "Life (9th ed.)" by Sadava et al. (2011)
- The level of depolarization that causes an electrically excitable membrane to fire an action potential.
Glossary of "Environmental Science: Earth as a Living Planet (2nd ed.)" by Botkin & Keller (1998)
- A point in the operation of a system at which a change occurs. With respect to toxicology it is a level below which effects are not observable and above which effects become apparent.
- Botkin, D. B. & Keller, E. A. 1998. Environmental Science: Earth as a Living Planet, 2nd ed. -xxxxii + 649 pp. John Wiley & Sons, Inc., New York.
- 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.