focal vs principal

focal

adj
  • Belonging to, concerning, or located at a focus. 

  • Limited to a small area. 

noun
  • The individual who is the focus of a study or review, when the study or review is based on that individual's interactions with others. 

  • The central or most important element of something; a focal element. 

  • A sign or similar type of marketing material designed to draw attention to special deals. 

  • An object that is used to focus concentration when performing magic. 

  • A representative of a group or class of people within an organizational system. 

  • An exemplar of a concept. 

  • A major point of interest; an attraction. 

principal

adj
  • Primary; most important; first level in importance. 

  • Chosen or assumed among a branch of possible values of a multi-valued function so that the function is single-valued. 

noun
  • The chief administrator of a school. 

  • A diapason, a type of organ stop on a pipe organ. 

  • The construction that gives shape and strength to a roof, generally a truss of timber or iron; or, loosely, the most important member of a piece of framing. 

  • A dancer at the highest rank within a professional dance company, particularly a ballet company. 

  • The primary participant in a crime. 

  • The first two long feathers of a hawk's wing. 

  • A security principal. 

  • A legal person that authorizes another (the agent) to act on their behalf; or on whose behalf an agent or gestor in a negotiorum gestio acts. 

  • The chief executive and chief academic officer of a university or college. 

  • The money originally invested or loaned, on which basis interest and returns are calculated. 

  • A partner or owner of a business. 

  • One of the turrets or pinnacles of waxwork and tapers with which the posts and centre of a funeral hearse were formerly crowned 

How often have the words focal and principal occurred in a corpus of books? (source: Google Ngram Viewer )