The term "arm" has several meanings, depending on its context.
1) In anatomy, arm refers to one of the two upper limbs of the human body, extending from the shoulder to the elbow joint and containing muscles that allow for movement and grasping objects.
2) Figuratively, "arm" can refer to a powerful supporter or backer, as in the phrase "with the backing of...".
3) In a more general sense, an arm is any extension or part of something, such as the branch of a tree or river.