Final answer:
A primary key is a unique attribute or combination of attributes in a database relation that identifies each record uniquely.
Step-by-step explanation:
A primary key is an attribute (or combination of attributes) whose value is unique across all occurrences of a relation. Its main purpose is to identify each record in a database table uniquely. The primary key can be a single field or a combination of multiple fields, which is known as a composite key.
It's important that these keys do not change in value, which is why they are carefully chosen when designing a database. Indexes, on the other hand, are used to speed up search queries but are not necessarily unique. An identifier is a broader term which can refer to any attribute that is used to identify a record, whether unique or not.