What is the best way to build definition without gaining weight or size? in other words, how do i develop lean muscle?

Lo-weight, hi-rep: You would like to fat-burn, which is best done just below the "cardio" levels, so use weights or can do isometric exercises to work at a moderate pace that allows you to continue for 30-45 min (not that you would start that long--work up to it).

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.