There are plenty of synonyms for the phrase "important thing", ranging from the mundane to the profound. At a basic level, the terms "essential", "key", and "critical" all convey a sense of necessity and importance. More grandiose synonyms might include "pivotal", "foundational", or "crucial". Some synonyms add a more emotional or personal dimension to the concept: "meaningful", "significant", or "vital" suggest that the important thing in question holds a particular value or significance to the speaker. Ultimately, the choice of synonym depends on the context and the tone desired: a legal document might employ "critical", while a romantic sonnet might go with "crucial".