Ma'anar ƙamus na kalmar "ƙasa" saitin ne wanda aka haɗa cikin wani saiti. Ma'ana, juzu'i wani yanki ne ko wani yanki na babban saiti wanda ya ƙunshi abubuwa kawai waɗanda kuma suke cikin babban saiti. Misali, saitin {2, 4, 6} wani yanki ne na saitin {1, 2, 3, 4, 5, 6}, saboda duk abubuwan da ke cikin rukunin suma suna nan a cikin babban saitin. Duk da haka, saitin {2, 4, 8} ba wani yanki ne na {1, 2, 3, 4, 5, 6} ba, saboda kashi 8 ba ya cikin babban saitin.