The last possible moment to do something.
The closing date for an application, a tax return, a tender process, homework, projects or registrations. Deadline literally means the pont in time that a process dies.
Some people do their work early, but it is very common to leave things until the deadline, the last moment. Teachers give deadlines to students to force them to organize their work schedules. For many people deadlines are beneficial as they force people out of a state of apathy. For others however deadlines can be counterproductive as the deadline only ends up stressing the student.
If you are going to miss a deadline, you should try asking for an extension. At school, university and work it is sometimes possible to obtain an extension to the deadline, however deadlines for things like voter registration, or the filing of tax returns cannot be extended.
For example: The deadline for voter registration is the twenty sixth of September.
This project must be handed in by Friday. The deadline is Friday at midday.
If you don't file your taxes before the deadline, you will get a fine.