critical - adjective

• marked by a tendency to find and call attention to errors and flaws
• at or of a point at which a property or phenomenon suffers an abrupt change especially having enough mass to sustain a chain reaction
• characterized by careful evaluation and judgment
• urgently needed; absolutely necessary
• forming or having the nature of a turning point or crisis
• being in or verging on a state of crisis or emergency
• of or involving or characteristic of critics or criticism

critical - thesaurus

decisive   vital