繼電器（英文名(ming)稱：relay）是(shi)一種電(dian)(dian)控(kong)制(zhi)器件，是(shi)當輸(shu)入量（激勵量）的(de)變化達(da)到規定要求時，在(zai)(zai)電(dian)(dian)氣輸(shu)出電(dian)(dian)路(lu)中使被(bei)控(kong)量發生預定的(de)階(jie)躍(yue)變化的(de)一種電(dian)(dian)器。它具(ju)有控(kong)制(zhi)系(xi)(xi)統(tong)（又稱輸(shu)入回(hui)路(lu)）和被(bei)控(kong)制(zhi)系(xi)(xi)統(tong)（又稱輸(shu)出回(hui)路(lu)）之間的(de)互(hu)動(dong)關系(xi)(xi)。通(tong)常應用于自動(dong)化的(de)控(kong)制(zhi)電(dian)(dian)路(lu)中，它實際上是(shi)用小電(dian)(dian)流(liu)去控(kong)制(zhi)大(da)電(dian)(dian)流(liu)運作的(de)一種“自動(dong)開關”。故在(zai)(zai)電(dian)(dian)路(lu)中起著自動(dong)調節(jie)、安(an)全(quan)保護、轉換(huan)電(dian)(dian)路(lu)等作用。