Wait . . . think carefully before you answer.
If you said the long limb below your waist (or equivalent), you'd be wrong! Well, okay, it would be correct in everyday conversation. But it is not anatomically correct.
In anatomical terms, the lower limb (lower extremity) is made of up of the thigh, leg, and foot. Anatomically, the leg is only that section of the lower limb just below the knee.
The same sort of thing happens with the arm. In anatomy, the entire upper limb (upper extremity) is not the arm. Only the section above the elbow is the arm. The section just below the elbow is the forearm.
One way to help you remember this is think of pieces of chicken. I realize that not everyone is a meat eater. But those who eat chicken know that it's often served in pieces. A chicken leg is not the entire lower limb--there's another piece just proximal to the leg: the thigh. Remember that, and it may be easier to keep the distinction straight in human anatomy.
Likewise, one can think of the two types of pieces often served in an order of chicken wings. The "drumette," which looks like a mini drumstick, is the arm. The "flat" or "wingette" is the forearm.