This is a term I borrow from Kirby Kid -- aka Richard Terrel.
Mechanics are the player initiated actions from controller inputs as designated by the game designers. These actions have effects on the gamestate in terms of the variables and dynamics of the gameplay system.
In fact, if we consider that gameplay is the part that defines and rules interactions between game elements based on user inputs, mechanics are actually atomical element of the gameplay.
I'm mostly building platformers here, so my primary mechanics are JUMP and RUN. I can modulate them with FLOAT. I don't quite have a shooting or fighting character, but it instead GRAB and THROW things.
As you have noted, I use all-caps when I want to obviously refer to gameplay actions (mechanics) within the text. I promise I won't shout too much.
Readme: KK: Mechanics & Abstractions (p.2)