What code do you use to define the constraint?

To define a constraint in programming, you typically use conditional statements or specific functions depending on the context. For example, in SQL, you might use CHECK constraints within a table definition like this: CREATE TABLE example (id INT, age INT CHECK (age >= 18));. In Python, you could use an if statement to enforce a constraint, such as if age < 18: raise ValueError("Age must be 18 or older."). The exact code will vary based on the programming language and the type of constraint being implemented.