When something absolutely has to be done and cannot be put off, use the adjective imperative.
Imperative is from Latin imperare, "to command," and its original use was for a verb form expressing a command: "Do it!" is an imperative sentence. The word is still used that way, but it's more commonly applied to something so pressing it cannot be put off: "It's imperative that we hire more workers if we want to complete the job on time." Imperative has more immediate force than pressing but less than urgent.