The hair-styling tool that uses heated ceramic plates to straighten hair is a flat iron.
Also called a straightening iron or hair straightener, a flat iron has two flat heated plates that clamp around sections of hair. The heat temporarily alters hydrogen bonds in the hair’s cortex, allowing the strands to be pressed into a straighter shape. Moisture can allow those bonds to reform, which is why humidity may bring back natural waves or curls.
Ceramic plates are widely used because they distribute heat relatively evenly, although some models use ceramic coatings, titanium, or other materials. The tool should not be confused with a curling iron, whose heated barrel creates curls, or hot rollers, which shape hair while it cools. A blow dryer uses moving heated air rather than clamping plates.
The basic idea predates modern electric styling tools. In 1909, Isaac K. Shero patented a straightener made from two heated flat irons pressed together. Earlier hair-straightening practices included heated metal combs, while some people in the 1960s even used clothing irons.