Hyperhidrosis means excessive sweating. It can be localised or affect the whole body.
Sweating is controlled by the brain, which sends signals along nerves called “sympathetic nerves” to the small sweat glands in the skin. These nerves are part of the “autonomic nervous system” which controls many unconscious body functions.
Increased sweating is a normal response to a rise in body temperature, and to emotions such as anxiety.
A treatment which reduces sweating is called an antiperspirant. This is different from a deodorant, which reduces odour, usually through an antibacterial effect. The two are often combined in the same product.