A deformity or malformation was an unnatural growth of an individuals body part.
Upon uncovering the cosmetic surgery of Captain Jonathan Archer and Malcolm Reed, General Gosis told the two that Doctor Temec believed their deformities were not the result of a surgeon as there were no scars. (ENT: "The Communicator")
A rumor existed about Xindi-Reptilian Commander Dolim, indicating that his daughter gave birth to a son, who was born with a deformity in his right arm. Although non-life-threatening, the deformity was enough to preclude future military service, something which Dolim was unwilling to abide. In response, Dolim had his own grandson poisoned. (ENT: "The Council")
Roger Korby defended his experiments with androids in a discussion with Captain James T. Kirk. He said that placing individuals into androids would eradicate diseases, deformities, and even fear could be programmed away. (TOS: "What Are Little Girls Made Of?")
The Srivani Alzen threatened Captain Kathryn Janeway that her species would continue their research with the crewmembers of the USS Voyager. When Janeway and her crew would make no further attempts to interfere, she assured that the fatality rate would be minimal, though there may be some deformities. (VOY: "Scientific Method")