Definition of "Main" :

noun: a principal pipe in a system that distributes water or gas or electricity or that collects sewage

noun: any very large body of (salt) water

adjective: (of a clause) capable of standing syntactically alone as a complete sentence

"The main (or independent) clause in a complex sentence has at least a subject and a verb."

adjective: most important element

"The main doors were of solid glass."

adjective: of force; of the greatest possible intensity

"By main strength."