The Collaborative International Dictionary of English v.0.48:

Fac \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.
   [1913 Webster]

The Free On-line Dictionary of Computing (19 January 2023):

FAC

   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)].