The Collaborative International Dictionary of English v.0.48:

Focal \Fo"cal\, a. [Cf. F. focal. See Focus.]
   Belonging to,or concerning, a focus; as, a focal point.
   [1913 Webster]

   Focal distance, or length, of a lens or mirror (Opt.), the
      distance of the focus from the surface of the lens or
      mirror, or more exactly, in the case of a lens, from its
      optical center.

   Focal distance of a telescope, the distance of the image of
      an object from the object glass.
      [1913 Webster]

WordNet (r) 3.0 (2006):

focal
    adj 1: having or localized centrally at a focus; "focal point";
           "focal infection"
    2: of or relating to a focus; "focal length"

Moby Thesaurus II by Grady Ward, 1.0:

66 Moby Thesaurus words for "focal":
   approaching, arch, asymptotic, banner, basic, bedrock, capital,
   cardinal, central, centralized, centred, centripetal, centrolineal,
   champion, chief, concentrated, concurrent, confluent, confocal,
   connivent, convergent, converging, crowning, dominant, essential,
   first, foremost, fundamental, great, headmost, hegemonic,
   indispensable, leading, life-and-death, life-or-death, magisterial,
   main, master, material, meeting, mutually approaching,
   of vital importance, overruling, paramount, predominant,
   preeminent, preponderant, prevailing, primal, primary, prime,
   principal, radial, radiating, ranking, ruling, sovereign, star,
   stellar, substantive, supereminent, tangent, tangential, topflight,
   uniting, vital



V.E.R.A. -- Virtual Entity of Relevant Acronyms (February 2016):

FOCAL
       FOrmula CALculator


The Free On-line Dictionary of Computing (30 December 2018):

FOCAL

   1. FOrmula CALculator.

   An interactive system written by Rick Merrill of DEC in 1969
   for PDP-5 and PDP-8.  It was a descendant of AID/JOSS.

   Versions: FOCAL-69, FOCAL-1971, FOCAL-11 (for PDP-11 under
   RT-11).

   (1994-12-21)

   2. Forty-One CAlculator Language.

   The programming language of the HP-41 calculator line.

   (1994-12-21)