What is the fastest way to gain muscle mass for definition?

Protein/resistance: Muscles need protein to grow, but the body needs fatty acids, vitamins, and minerals as essential nutrients. Don't overdo proteins because they break down into purines and too much can lead a build up of uric acid and a disease called gout. Healthy sources of protein are beans, nuts, and fish like wild salmon. Resistance and strength training build muscle as well. I don't advocate protein powder.

Exercise/Nutrition: Weight training generally helps to shape and tone muscles while cardiovascular exercise burns the fat around the muscles so you can see them better. Lighter weights with higher reps are better for tone, and heavier weights with fewer reps would be better for size and strength. So you need higher rep exercises, regular cardio and diet of lean protein and complex carbs.