A cousin — the child of one's uncle or aunt. Both male and female cousins are commonly called "itoko" in Japanese.
子供の頃、いとこと毎夏一緒に遊んだ。
When I was a child, I played with my cousin every summer.
いとこは来月結婚する予定だ。
My cousin is getting married next month.
従姉妹（じゅうしまい）は漢字表記の一つで、日常では「いとこ」と書くことが多い。
従姉妹 is one kanji notation; in everyday use people usually write いとこ.