Tekun Caspian babban tafkin ruwan gishiri ne, wanda ke tsakanin Turai da Asiya, yana iyaka da Rasha daga arewa, Kazakhstan a arewa maso gabas, Iran a kudu, Azarbaijan a yamma, da Turkmenistan a kudu maso gabas. Ita ce ruwa mafi girma a cikin ƙasa a duniya, wanda ke da faɗin fili kimanin mil murabba'in 143,000 (kilomita murabba'in 371,000). Sunan "Caspian" an yi imani da cewa ya samo asali ne daga tsoffin mutanen Caspian, mutanen da suka rayu a yankin da ke kusa da teku.