The abdomen was the part of a humanoid body between the thorax and the pelvis. The abdominal cavity was located here as well as several organs.
Another term for cramps was "post-prandial upper abdominal distension". (Star Trek IV: The Voyage Home)
In 2153, Phlox prescribed Charles Tucker Aldebaran mud leeches against insomnia, of which he was supposed to place one on his chest and one on his abdomen. (ENT: "Anomaly")
In 2268, Leonard McCoy was tortured by Vians, which caused many injuries, including bleeding in his chest and abdomen. (TOS: "The Empath")
In 2367, K'Ehleyr suffered from multiple stab wounds in her chest and abdomen after being attacked by Duras. (TNG: "Reunion")
In 2372, Samantha Wildman experienced a sharp pain in her abdomen due to the unborn Naomi Wildman shifting position. (VOY: "Deadlock")
In 2373, according to The Doctor, the abdominal organs of a deceased Borg drone were unremarkable. (VOY: "Unity")
According to Neelix in 2377, food laced with Talaxian wormroot caused abdominal spasms, which increased in severity for thirty to forty hours. (VOY: "Critical Care")