Updated: Nov 19, 2020
I'm about to give you the one exercise that will give you the most arm definition.
Really, of course, there's no specific answer, and I get this question a lot. First, you have to get lean to be able to see the definition. Body fat is what prevents you from seeing the shape and lines of muscles. Nutrition and how you're eating is most important.
But here's the quick take home! Develop shoulders well and the rest of your arms, biceps and triceps will look better and more defined. Work the posterior, anterior and medial deltoid muscles and watch how much it brings the look of your arms all together. Take a look at the graphic to see where the deltoid muscles are and how to work them. IMPORTANT - the amount of weight used and the rep range can be different for each person relative to the exercise being performed, whether male or female, and your experience strength training. Message me if you need help figuring that out.