Synonyms: urgent, exigent, pressing, imperative
These adjectives mean compelling immediate attention. Urgent often implies that a matter takes precedence over others: "For I ride on an errand most urgent, and with the first light of morning we must go" (J.R.R. Tolkien).
Exigent and pressing suggest an urgency that requires prompt action: "When once disease was introduced into the rural districts, its effects appeared more horrible, more exigent, and more difficult to cure, than in towns" (Mary Shelley). "The danger now became too pressing to admit of longer delay" (James Fenimore Cooper).
Imperative implies a need or demand whose fulfillment cannot be deferred: "The stricken countries of Europe needed everything and could afford to buy nothing. Financial help was imperative" (David McCullough).