the point in time at which something must be completed
"The project manager reminded everyone that Friday is the deadline for submitting their final reports."
A time limit in the form of a date on or before which something must be completed.
"The project deadline is set for next Friday, so we need to finish all revisions by then."
In plain English: A deadline is the final time by which something must be finished or submitted.
"The teacher reminded us that the project deadline was due at the end of the week."
Usage: A deadline is a specific date or time by which a task must be finished, not the moment you actually submit your work. You should use this noun to mark the final opportunity for completion rather than referring to the act of meeting that limit.
To render an item non-mission-capable; to ground an aircraft, etc.
"The maintenance crew issued a deadline on the fighter jet after discovering a critical hydraulic leak that would prevent it from taking off for combat missions."
In plain English: To reach a deadline means to arrive at a place by the specific time you promised.
"The manager will deadline the report by Friday afternoon."
Usage: Do not use "deadline" as a verb in everyday conversation; it is technically correct only in aviation and military contexts where it means to ground a vehicle or declare equipment unserviceable. In general usage, simply say that you are meeting the deadline or missing it rather than attempting to verbify the noun.
The word deadline originally described fixed lines used by anglers before shifting in America to mean an uncrossable prison boundary. During the Civil War, this specific usage evolved into its modern sense of "due date," referring to the moment after which a prisoner would be executed for crossing that line.