Refers specifically to an older female cousin. The kanji 従姉 contrasts with 従妹 (younger female cousin) and 従兄 (older male cousin). In speech, いとこ is used without gender/age distinction.
See also: 従妹
My older female cousin lives in Tokyo.
I went shopping with my older female cousin.
Kanji spelling specifically for an older female cousin. The reading いとこ is a jukujikun (special kanji reading).
The kanji 従 (juu) means 'secondary' or 'cousin', and 姉 (shi) means 'older sister'. The reading いとこ is a jukujikun, a native Japanese word assigned to these kanji.