You can’t train for definition. Most people, when referring to definition training, mean long, arduous hours of low-intensity work, which does not build muscle size but rather endurance or cardiovascular efficiency. To build size, you must do just the opposite, which means that training must be intense, infrequent and relatively brief.
One of the basic concepts of exercise physiology is specificity, which means that your body will adapt to achieve only one of those goals but not both. We all possess what stress physiologist Hans Selye referred to as adaptation energy, and this adaptation energy must be spent 100 percent for building either size/strength or endurance.
As a result, it would appear that you could more efficiently achieve size with definition if, instead of dividing that energy between the two, you trained specifically for size and simply reduced your caloric intake.
‘Peter Sisco and John Little, Power Factor Training