In everyday English, you would call the arm the part of the body that extends from the shoulder and ending at the fingertips. If you are studying in the medical field, each part of that arm is broken down into more specific terms.

I agree with Crystal, 100%, though I would add that context is important. If I say, "My arm hurts," we understand that I'm talking about the length from my shoulder to my wrist. If it was my hand, I would say, "My hand hurts," never "My arm hurts." So when you're specifically talking about limbs, as in "I have two arms, two legs, and a head," we're referring to the shoulder til the ends of the fingertips (just like "leg" is referring to the hip til the ends of the toes). But if you're referencing something specifically about the appendage, it can mean shoulder to wrist. Another example, "I got a tattoo on my arm." We know they didn't get the tattoo on their hand. We don't count anything past the wrist as "arm" in these instances.

In some languages, I know it's common to say arm for the whole part that extends from the shoulder to the fingertips. Personally, when I hear 'arm' I only think of the part that extends from the shoulder down to the wrist. In everyday English, “arm” can have two meanings, which is probably why this is confusing. Strictly speaking, the arm is the part from your shoulder to your wrist. The part from the elbow to the wrist is the “forearm,” and the part from the wrist to the fingertips is the “hand.” However, in everyday conversation, we often use “arm” to mean the whole limb, including the hand. For example, “Raise your arm” normally means raising your whole arm and hand. If you need to be anatomically precise, the whole thing from the shoulder to the fingertips can be called the “upper limb,” but this is mainly a medical or technical term. So, in normal conversation, it is completely natural to think of your “arm” as including your hand, even though anatomically the hand is a separate part. I know it's confusing but I hope that helps.