A second cousin is the grandchild of one's grandparent's sibling.
A second cousin: the child of a parent's cousin, sharing a great-grandparent. Also said as またいとこ or written 再従兄弟 (and several other kanji forms by sex), though the kana form is most common.
Similar words
いとこ: (first) cousin — the child of a parent's sibling; one generation closer than はとこ
親戚: relative — the general term for any extended-family relation