To swelter means to feel uncomfortably hot, usually because of humid weather or being in a stuffy, poorly ventilated space. When you're sweltering, you're sticky, sweaty, and miserable, wishing desperately for a breeze or some air conditioning.

You might swelter on a humid August afternoon when the air feels thick and heavy, or swelter in a crowded gym during an assembly when too many people are packed into a space with no windows open. The word captures that oppressive, draining feeling of heat that seems to press down on you from all sides.

Sweltering is the adjective form: a sweltering summer day or sweltering heat. Unlike words like “warm” or “hot,” which can be pleasant, sweltering always suggests discomfort. A cozy fire makes you warm; a broken air conditioner on a July afternoon makes you swelter.

The word often appears in weather reports and historical accounts. Soldiers in thick wool uniforms sweltered during summer campaigns. Factory workers sweltered before buildings had modern cooling systems. Today, athletes swelter during intense outdoor practices in humid conditions, which is why coaches schedule water breaks and watch for signs of heat exhaustion.