A sigmoid function is a generic term refering to S-like functions that have a step-like nature, but are smooth without the discontinuity of the threshold function. This maes them more emnable to use in machine learning as, in general, continuity aids learning.
Used on Chap. 6: page 115; Chap. 7: page 146; Chap. 9: pages 191, 192; Chap. 10: page 210; Chap. 21: page 523
Also known as sigmoid
Sigmoid function smoothly limits extreme values