Correct spelling, explanation: the words oldest and eldest are both correct and the choice between them depends on the context and the noun you are describing. The first one is used when comparing the ages of people or things. It is the superlative form of old. The second one is specifically used to describe the oldest sibling within a family. It is not used to compare the ages of things or non-sibling individuals. Oldest and eldest are very similar in structure and meaning, so be careful not to confuse them. Pay attention to the context of the sentence to avoid making a mistake.

Definition of oldest:
superlative adjective, used to refer to someone or something that has existed for the longest period of time when comparing multiple people or things
She is the oldest person in the family.
Among the books on the shelf, that one is the oldest.

Collocations with oldest:
Some most commonly used collocations include:
1. Oldest child: refers to the firstborn child in a family.
As the oldest child, she often takes on a leadership role among her siblings.
2. Oldest profession: refers to the euphemistic term for prostitution, often used in historical or literary contexts.
The oldest profession has existed in various forms throughout human history.
3. Oldest trick in the book: refers to a well-known or commonly used deception or tactic.
Trying to flatter your way out of trouble is the oldest trick in the book.
4. Oldest living person: describes the individual who has lived the longest among the current population.
The world’s oldest living person recently celebrated her 115th birthday.

Correct spelling, explanation: oldest and eldest both refer to the category of being the oldest in terms of age or seniority, but they are used in slightly different contexts. Oldest is a comparative adjective used when comparing the ages of people, things, or entities without specific reference to familial relationships. It can be used in a broader sense to compare the ages of anything, not just siblings or family members. Eldest is also a comparative adjective but is specifically used within the context of family relationships to describe the sibling who was born first or has the highest seniority in the family.