Definition
The total amount of work done, counted as hard sets per muscle per week, or as sets times reps times load.
Two definitions are in use. Set count (hard sets per muscle per week) is the practical one for planning. Tonnage (sets x reps x kg) is useful for tracking a single lift over time.
For muscle, 10 to 20 hard sets per muscle per week covers most people; beginners at the low end, and more than 25 rarely helps. Volume is the main lever for size, intensity the main lever for strength. Count only sets within about four reps of failure; the rest is warm-up.
Raise it slowly, two to four sets per muscle per week across a block, and then deload. Volume is also the first thing to cut when a client is short on sleep or time.