“Main Clause, also principal clause. A clause in a sentence to which other clauses are subordinated, and which is not itself a subordinate clause. In a simple sentence, the main clause is the entire sentence. In a compound sentence, there is more than one main clause. Complex sentences contain a main clause and one or more subordinate clauses.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.