\Fac\ (f[a^]k), n. [Abbrev. of facsimile.] A large ornamental letter used, esp. by the early printers, at the commencement of the chapters and other divisions of a book. --Brande & C.
Functional Array Calculator. An APL-like language, but purely functional and lazy. It allows infinite arrays.
["FAC: A Functional APL Language", H.-C. Tu and A.J. Perlis, IEEE Trans Soft Eng 3(1):36-45 (Jan 1986)].
Webster's 1913 Dictionary