Elastic
A stretchable band or cord, usually rubber or spandex core wrapped in woven or knit fibers, used for waistbands, cuffs, and stretch closures.
Most sewing elastic is a core of rubber or spandex filament wrapped or woven with another fiber (cotton, polyester) around it, which is what lets it stretch and recover instead of just being a bare rubber band. It comes in a genuinely confusing range of types once you start looking closely — braided, which narrows when stretched and is the cheapest, general-purpose option; knitted, which keeps its width under stretch and doesn't narrow, better for waistbands that'll be sewn directly through; woven, the firmest and least likely to roll, best for heavier-support waistbands.
The mistake I made constantly as a beginner: buying elastic by width alone and grabbing whatever braided elastic was cheapest, then wondering why a waistband looked twisted and uneven after a few wears. Braided elastic narrows and can roll inside a casing precisely because it's not stable under tension — for anything stitched directly through the elastic rather than run through a casing, knitted or woven is the better call every time.
I always cut elastic a bit shorter than the actual body measurement, not equal to it or longer — elastic needs to be under some tension to actually hold anything up, so measuring for a snug (not tight) fit before cutting matters more than following a chart.