From WordNet (r) 3.0 (2006) :
natural number
n 1: the number 1 and any other number obtained by adding 1 to
it repeatedly
From The Free On-line Dictionary of Computing (30 December 2018) :
natural number
An integer greater than or equal to zero. A
natural number is an isomorphism class of a finite set.
(1995-03-25)