Versine

$$\mathop{\rm vers}(z)\equiv 1-\cos z,$$

where $\cos z$ is the Cosine. Using a trigonometric identity, the versine is equal to

$$\mathop{\rm vers}(z)=2\sin^2({\textstyle{1\over 2}}z).$$

See also Cosine, Coversine, Exsecant, Haversine

References

Abramowitz, M. and Stegun, C. A. (Eds.). Handbook of Mathematical Functions with Formulas, Graphs, and Mathematical Tables, 9th printing. New York: Dover, p. 78, 1972.