The thickening of muscle fibres in response to training — the physiological process behind 'building muscle'.
Hypertrophy is driven mainly by two things: mechanical tension, meaning work close to your limit, and sufficient weekly volume. A rough guideline is 10–20 hard sets per muscle group per week; the classic range is 6–15 reps at 65–80% of 1RM, though newer work shows even 30-rep sets work as long as you approach failure. Two factors matter more than program choice: progression across weeks and months, and a slight calorie surplus with enough protein (roughly 1.6–2.2 g per kilogram of body weight). No building material, no building, however good the plan. And muscle growth is slow — a few kilos in the first year is realistic, not ten.