Exsecant

$$\mathop{\rm exsec}\nolimits x\equiv\sec x-1,$$

where $\sec x$ is the Secant.

See also Coversine, Haversine, Secant, Versine

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.