Answer:
A field with a unique value
Explanation:
A primary key is a field (or a collection of fields) that contain a unique data to identify a precise row in a given table. This is used for data retrieval but also to fully identify the row to be edited in case of update.
It can be as simple as an auto-increment counter, a serial number or an email address for example.
It can also be complex and take into account a series of fields, like a combination of a date and a userID, as a simple example. In such case, the date field by itself would occur many times (many entries per day), and the userID would happen also many times (many daily entries by the userID), but the pair (date/userID) would be unique.
