A transition constraint is a way of enforcing that the data does not enter an impossible state because of a previous state. For example, it should not be possible for a person to change from being "married" to being "single, never married". The only valid states after "married" might be "divorced", "widowed", or "deceased".
Transition constraint is commonly used in database models such as relational databases.
References
Transition constraint Wikipedia(Text) CC BY-SA