definition of pin
pin. 1  a small, slender, often pointed piece of wood, metal, etc., used to fasten, support, or attach things. 2  a short, slender piece of wire with a point at one end and a head at the other, for fastening things together. any of various forms of fasteners or ornaments consisting essentially or partly of a pointed or penetrating wire or shaft (often used in combination): a jeweled pin.