1. the number 1 and any other number obtained by adding 1 to it repeatedly;

WordNet (r) 3.0 (2006):

natural number n 1: the number 1 and any other number obtained by adding 1 to it repeatedlyThe Free On-line Dictionary of Computing (30 December 2018):

natural numberAn integer greater than or equal to zero. A natural number is an isomorphism class of a finite set. (1995-03-25)