Two points lying on a circle actually define two arcs. The shortest is called the 'minor arc' the longer one is called the 'major arc'. In the figure above, if you were to refer to the 'arc AB' you could mean either one. Typically, if you don't specify which, readers will assume you mean the minor (shortest) arc. If there is a possibility of confusion, you should state which one you mean.

Another way to avoid confusion is to have another point on the arc and use all three to define it. For example 'arc AQB', would not be in doubt since the point Q would lie on only on one of the two possible arcs.