sigmoid vs sigmoidal

sigmoid

adj
  • Curved in two directions, like the letter "S", or the Greek ς (sigma); having a serpentine shape. 

  • Relating to the sigmoid colon. 

  • Exhibiting logistic growth; having a graph that accelerates until it reaches a carrying capacity. 

sigmoidal

adj
  • sigmoid; S-shaped. 

  • Characterized by a sigmoid curve or function 

noun
  • A sigmoidal function or curve 

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