-certain properties and constraints on data elements -integrity constraints can be enforced by dbms ex. -date of birth can't be in the future  -ssn should be 9 digits
data meet certain constraints, such as no alphabetic characters in a tax file number (tfn) number field.
We think this planet formed with its star 12.713 billion years ago , when the [ Milky Way ] galaxy was . . . just in the process of forming .