The plane that divides the body into left and right halves, containing every movement that travels forward or backward.
Picture a sheet of glass running from the front of the head to the back, splitting the body down the middle. Anything that moves along that sheet is sagittal: squats, deadlifts, bench press, rows, curls, lunges, running.
It is where almost all barbell training lives, which is the practical point. A client whose whole week is squat, bench, deadlift and row has trained one plane five days running, and the gaps show up as weak side-to-side control and weak rotation.
The fix is small. Add one frontal-plane movement (a lateral lunge or a side plank) and one transverse-plane movement (a cable chop or a Pallof press) to two sessions a week, two sets each. That is ten minutes and it covers the other two planes without redesigning the program.