A default constraint enables you to define the value that will be supplied for a column whenever a user fails to enter a value. For example, in a table with a column called payterms, you can instruct your database server to enter "???" or "fill in later" if the user leaves it blank.
In database diagrams, you define a default constraint as a property of a column in your table. You define this type of constraint for a column by specifying a default value inside a table in Column Properties view. Be sure to specify the constraint with the correct delimiters. For example, strings must be surrounded with single quotes.
For more information about defining default constraints, see Setting Column Properties.