Page number.
I don't have the book handy. How's D&D Beyond for a reference?
"Each of a creature's abilities has a score, a number that defines the magnitude of that ability. An ability score is not just a measure of innate capabilities, but also encompasses a creature's training and competence in activities related to that ability."

Player's Handbook
Everything a player needs to create heroic characters for Dungeons & Dragons the world’s greatest roleplaying game.
