Which definition best matches "urgent"?

"Urgent" is most accurately defined as "requiring immediate action." This word conveys a sense of immediacy and importance that demands prompt attention or resolution. In many contexts, urgency suggests that the situation at hand cannot be postponed or ignored, often because it can lead to negative consequences if not addressed quickly.

The other definitions do not align with the meaning of "urgent." For instance, something that is "not important" would not necessitate immediate action, and describing a situation as "casual and relaxed" or "routine and unchanging" implies a laid-back demeanor that conflicts with the intense pressure felt in urgent situations. Therefore, the definition focusing on the need for immediate action is the most appropriate and fitting for the term "urgent."