A HAIRPIN curve is a type of road turn that is characterized by a sharp U-shaped bend. It is also known as a switchback or a reverse curve. The term 'HAIRPIN' comes from the resemblance of the shape to a HAIRPIN or bobby pin. HAIRPIN curves are often used in mountainous terrain, where a long, gradual curve would take up too much space. They are also used in race tracks and other areas where a sharp turn is desired. The crossword solution for the term 'HAIRPIN curve' is ESS, which stands for 'extreme sharp turn'.