Distension is the swelling itself — the state of being stretched outward by pressure from inside. You see it most in medical English: abdominal distension, gastric distension. It can be gas, fluid, a big meal, or something a doctor needs to check. The word names the condition, not the verb. British spelling often keeps the s (distension); American writing often uses distention. Both point to the same thing. It is a chart-and-textbook noun. Ordinary talk says bloating or swelling. It is not extension, which is about making something longer in space or time. And it is not a mild dislike (that is distaste); only the letters at the start look similar.

膨脹、鼓脹（尤指腹脹；美式拼法常見 distention）