Final answer:
An aggregate function combines data from multiple rows together to perform a calculation and return a single value. Common examples include SUM, COUNT, AVG, MIN, and MAX. It is used in SQL queries, often with the GROUP BY clause.
Step-by-step explanation:
An aggregate function in the context of databases like SQL is a type of function used to perform a calculation on a set of values, and then return a single value. Specifically, it combines data from multiple rows together. This means during a query, the function takes values from multiple rows and aggregates them into a single output.
For example, the SUM function adds up all the values in a particular column. Other common aggregate functions include COUNT, AVG (average), MIN (minimum), and MAX (maximum). These functions can be applied to table columns as well as to calculated columns, contrary to one of the options presented in the question. Lastly, when using aggregate functions with the GROUP BY clause, it is necessary that every column in the SELECT statement either be part of an aggregate function or be listed in the GROUP BY clause, ensuring that the query's result set is grouped properly.