cousin
A child of your aunt or uncle in your family.
A cousin is your aunt or uncle's child, making them part of your extended family. If your mom has a sister, that sister's kids are your cousins. If your dad has a brother, that brother's kids are your cousins too.
Cousins occupy an interesting spot in family life. They're relatives you're connected to by blood, but unlike siblings, you don't usually live with them. Some cousins grow up practically like brothers and sisters, seeing each other constantly at family gatherings, holidays, and vacations. Other cousins might live far away and only connect occasionally. Either way, cousins often share family traits: maybe you and your cousins all have the same distinctive laugh, or you all inherited your grandmother's curly hair.
The children of your first cousins are your first cousins once removed (a confusing term that basically means they're one generation away). Your parents' first cousins are also your first cousins once removed. Meanwhile, second cousins are the children of your grandparents' siblings' grandchildren, which means you share great-grandparents with them instead of grandparents.
People sometimes use “cousin” more loosely to describe any relative who isn't a sibling, parent, aunt, or uncle, especially at big family reunions where the exact relationships get complicated. The word can also describe things that are related or similar, like how basketball and netball might be called cousin sports.