A vitamin is any complex chemical which is not an amino acid that is either found in the diet or produced by the body the absence of which results in the development of a particular illness. This excludes elements such as iron and zinc which are not complex chemicals.
Some of the better known vitamins are:
Vitamin A, which is necessary for the proper functioning of certain light absorbing cells in the retina, the absence of which causes night blindness.
Vitamin B6, which has a role in the transmission of nerve impulses, the absence of which causes beri-beri.
Vitamin C, which plays a role in the formation and maintenance of connective tissues, the absence of which causes scurvy.