To distend means to swell or stretch outward from internal pressure, like a balloon filling with air or your stomach expanding after a big meal. The word describes something becoming larger and rounder because something inside is pushing outward.

You see distension in everyday life: a chipmunk's cheeks distend when stuffed with acorns, a water balloon distends as you fill it from the tap, or a snake's body distends after swallowing its prey whole. Medical professionals use the word too. They might notice that a patient's abdomen has become distended, meaning it's swollen and tight from gas, fluid, or other internal pressure.

The word carries a sense of stretching beyond normal size, often uncomfortably. A properly inflated bicycle tire isn't distended, it's just inflated. But if you kept pumping air into it until it bulged unnaturally, then it would be distended and probably about to pop. The key idea is expansion caused by something pushing from the inside out, making the outer surface stretch taut and round.