The fact that muscles will improve in strength and conditioning in response to resistance training is called the

The fact that muscles will improve in strength and conditioning in response to resistance training is called MUSCLE HYPERTROPHY. 

This occurs because during resistance training the muscles develop very small tears, called micro-tears, that eventually heal and become scar tissues. As the scar tissues continue to increase, muscle fibers are said to bulk up creating that big and muscular physique that you see in people who are into resistance training.