General terms:
* Independent clause - This is the most accurate and common synonym.
* Principal clause - This emphasizes its importance within the sentence.
* Superordinate clause - This highlights its role as the dominant element in a complex sentence.
More specific terms:
* Matrix clause - This is used in formal linguistics to describe the main clause in a complex sentence that contains embedded clauses.
* Root clause - This focuses on the clause being the starting point or foundation of the sentence.
Informal terms:
* Main sentence - This is less formal and might be used in casual conversation.
* The core of the sentence - This emphasizes the central meaning of the clause.
The best choice will depend on the specific context and the desired level of formality.