Define percent natural abundance.
The percentage amount of each isotope of an element in a naturally occurring sample of the element.
For example, Neon Ne, there are 3 different Ne isotopes:
10 neutrons, 11 neutrons, and 12 neutrons. 10 makes up 90.48% of random natural samples, 11 makes up 0.27%, and 12 makes up 9.25%.