Definition
A training block of roughly three to six weeks built around one goal and usually ending in a deload.
A typical hypertrophy mesocycle: week one 3 sets at 3 RIR, week two 3 sets at 2 RIR, week three 4 sets at 2 RIR, week four 4 sets at 1 RIR, week five deload. Each week is slightly harder than the last, then the fatigue is cleared.
Length depends on training age and life. Beginners and busy clients do well on four weeks; advanced lifters can push six. Judge the block by the same lifts at the same rep range from week one to the last loading week.
A mesocycle is also the natural unit for changing exercises: swap a movement between blocks, not in the middle of one.