The elder of two people is the one who was born first.
If you have a sister or brother who was born before you, you can refer to them as your elder sister or brother.
The eldest of a group of people, especially the brothers and sisters in a family, is the one who was born first.
Elder and eldest are slightly formal, and many people do not use them at all. Instead of 'elder' and 'eldest' you can use older and oldest.
You can use older and oldest in some ways in which you cannot use 'elder'. For example, you can use older after be, get, or grow, and in front of than.
You cannot use 'elder' in any of these ways.
You can also use older and oldest to talk about things.
You cannot use 'elder' or 'eldest' to talk about things.
| Noun | 1. | eldest - the offspring who came first in the order of birth |
| Adj. | 1. | eldest - first in order of birth; "the firstborn child" first - preceding all others in time or space or degree; "the first house on the right"; "the first day of spring"; "his first political race"; "her first baby"; "the first time"; "the first meetings of the new party"; "the first phase of his training" |