Question

In a relational database, is a tuple an entire single row within a table, or is it just one cell?

Is it part of the schema or part of the data?

Answer 1

Tuple stands for an entire row; more precisely it stands for a row with a specific set of columns. More like a function for every row.
More info: http://www.tomjewett.com/dbdesign/dbdesign.php?page=tables.php

Answer 2

A tuple represents one row of the relational database or a row in the result of a specific query. It is part of the schema, as it simply encapsulates the data.