* Technically, no: A limb is typically defined as an appendage used for locomotion, like arms and legs. While a monkey's tail can be used for balance and grasping, it's not primarily used for walking or running.
* Functionally, sometimes: Some monkey species, like prehensile-tailed monkeys, have tails that are so strong and flexible that they can act like a fifth limb. They use their tails to hang, swing, and even grasp objects.
So, while a monkey's tail isn't a limb in the strict anatomical sense, it can definitely function as one in some species.