A training split that divides the body into pushing muscles, pulling muscles and legs, run over three or six sessions a week.
Push day is chest, shoulders and triceps. Pull day is back and biceps. Leg day is quadriceps, hamstrings, glutes and calves. Every muscle that works together is trained together, so nothing gets hit two days running by accident.
Run three days a week it gives each muscle one session; run six days it gives two, which is where the split earns its reputation. A workable six-day week: push, pull, legs, rest, push, pull, legs, with the second rotation lighter and higher rep than the first.
It suits intermediate clients who can train five or six days and want the volume. It suits a three-day-a-week client less well than an upper/lower split, because once-weekly frequency per muscle is below what most people respond best to.