A cousin: the child of one's aunt or uncle. Usually written in kana as いとこ; kanji forms distinguish gender (従兄弟 male, 従姉妹 female) but in everyday speech いとこ is used.
幼い頃、夏休みはいつもいとこと遊んだ。
When I was little, I always played with my cousin during summer vacation.
いとこの結婚式に招待されたので、久しぶりに会えるのが楽しみだ。
I was invited to my cousin's wedding, so I'm looking forward to seeing them after a long time.
彼はいとこにあたるが、年が離れているのであまり親しくない。
He's my cousin, but we're not very close because there's a big age gap.