A hair straightener, also known as a flat iron, is a handheld styling tool designed to smooth and straighten hair by applying controlled heat to the strands. It typically consists of two heated plates that clamp together, allowing you to glide through sections of hair to eliminate frizz, curls, or waves. Modern versions often include features like adjustable temperature settings (up to 450°F for thick hair), ceramic or titanium plates for even heat distribution, and safety shut-offs to prevent accidents.