A deadline was a set time by which an action had to be accomplished.
In 1893, when asked, a newspaper reporter said his deadline was 5:00 PM. Hearing this, Samuel Clemens said he would have a story for him by 4:30. (TNG: "Time's Arrow, Part II")
In 2368, Captain Jean-Luc Picard stated the necessity for Romulan involvement in a conflict to be exposed before the thirteen hour deadline was up. (TNG: "Redemption II")
In 2369, Doctor Farallon stated that she was unsure if the particle fountain would be operating at full capacity within the 48-hour deadline, but that it would be close. (TNG: "The Quality of Life")