Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : Free On-Line Dictionary of Computing

FAC
     
        Functional Array Calculator.  An {APL}-like language, but
        {purely functional} and {lazy}.  It allows infinite {array}s.
     
        ["FAC: A Functional APL Language", H.-C. Tu and A.J. Perlis,
        IEEE Trans Soft Eng 3(1):36-45 (Jan 1986)].