In a relational database management system (DBMS), a tuple is a basic unit of data that represents a single instance of a relation or table in the database. It is also known as a record or row. Each tuple contains a set of values or attributes that correspond to the columns or fields of the relation.

For example, in a table of student records, each row or tuple would represent a single student and contain attributes such as name, age, gender, and marks.

It should be noted that tuples are mostly seen in relational databases management systems (RDBMS) as RDBMS works on the relational model (tabular format). In an RDBMS, a tuple is unordered.

In summary, a tuple in a database is a row with linked data about a certain entity, and it represents a single instance of a relation or table in the database.