functional vs principal

functional

adj
  • Having semantics defined purely in terms of mathematical functions, without side-effects. 

  • Only for functional purposes, notably in architecture 

  • Useful; serving a purpose, fulfilling a function 

  • Of a disease, such that its symptoms cannot be referred to any appreciable lesion or change of structure; opposed to organic disease, in which the organ itself is affected. 

  • In good working order. 

noun
  • A function that takes a function as its argument; More precisely: A function whose argument varies in a space of (real or complex valued) functions and whose value belongs to a monodimensional space. An example is the definite integration of integrable real functions in a real interval. 

  • A scalar-valued linear function on a vector space. 

  • An object encapsulating a function pointer (or equivalent). 

principal

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

  • Primary; most important; first level in importance. 

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 functional and principal occurred in a corpus of books? (source: Google Ngram Viewer )