Eldest usually means the oldest of the children in a family. If you have two younger siblings, you're the eldest child. The eldest brother or eldest sister is the one born first, before all the others.

People often use eldest when there are at least three people being compared, but it can also be used when there are only two. In a family with two kids, many people say older or younger, but some might still say eldest. In a family with three or more, one child is clearly the eldest.

Being the eldest often comes with certain expectations. Eldest children sometimes help look after younger siblings, set examples, or take on more responsibilities. In some cultures and throughout history, the eldest son or eldest daughter held special roles in the family, like inheriting the family farm or business.

The word has a slightly formal feeling to it. You might say “my older sister” in casual conversation but write “the eldest daughter” in a story or a formal description.