Refers specifically to a male cousin. The female counterpart is 従姉妹 (いとこ). In modern Japanese, いとこ is often used without gender distinction in kana.
See also: いとこ
He is my cousin.
I attended my cousin's wedding.
He and I are cousins on my father's side.
She is my cousin.
Kanji spelling for male cousin; rarely used in practice. The reading いとこ is a jukujikun.