NOT NULL in SQL is a constraint applied to a column in a table, ensuring that the column cannot have a NULL value. When a column is defined with the NOT NULL constraint, every row must have a valid, non-null value in that column. This ensures data integrity by preventing missing or unknown values where they are not allowed.

For example, if a name column is defined as VARCHAR(50) NOT NULL, any attempt to insert a row without a name or with a NULL value will result in an error.