Wiktionary
exsecant
n. (context trigonometry English) The trigonometric function sec(''x'') − 1. Abbreviation: '''exsec'''
Wikipedia
Exsecant
The exsecant (exsec, exs) and excosecant (excosec, excsc, exc) are trigonometric functions defined in terms of the secant and cosecant functions. They used to be important in fields such as surveying, railway engineering, civil engineering, astronomy, and spherical trigonometry and could help improve accuracy, but are rarely used today except to simplify some calculations.