Kashin baya shine dogon, cylindrical dam na zaruruwan jijiyoyi da nama mai alaƙa wanda ke rufe kuma an kiyaye shi a cikin ginshiƙin vertebral (ko kashin baya) na dabbobin kashin baya, gami da mutane. Sashe ne mai mahimmanci na tsarin juyayi na tsakiya wanda ke da alhakin watsa siginar hankali da motsi tsakanin kwakwalwa da sauran jiki. Kashin baya kuma yana taka muhimmiyar rawa a ayyukan reflex da kuma sarrafa ayyukan da ba na son rai ba kamar numfashi, bugun zuciya, da narkewa.