Other forms: mains
Main means the most important one among several — the main road, the main reason, the main course. It goes before a noun and almost never after one.
That position rule is worth having: you can say the main problem, but not the problem is main. Use chief, principal or primary in more formal writing; main is the everyday word. The noun is different: the mains are the public supply of water, gas or electricity, and on the mains means connected to it, which is British and very common. Note mainly, the adverb, meaning mostly, and the compounds mainland, mainstream and mainstay.
1adjectivemainLargest or most important of several.
“The main entrance is round the other side.”
2nounpublicThe public supply of water, gas or electricity.
“The house was not on the mains until 1975.”
3nounlargeA large pipe or cable carrying that supply.
“A burst main closed the road for two days.”
Too few real examples of this word to estimate which sense is commonest.
main is a degree of importance.
Also accepted in games: primary, chief
Main once meant strength. Old English mæġen was power or might, from the root megh, to be able, which also gives may. The main road is the strong one.
Proto-Indo-European
*megʰ-
Middle English
mayn
English, today
main
From Middle English mayn, main, maine, mæin, meyn, from main (see further at etymology 2); compare Old English mæġen (“strong, main, principal”) used in combination and Old Norse megn, megenn (“strong, main”). The word is cognate with Old High German megīn (“strong, mighty”) (modern German Möge, Vermögen (“power, wealth”)), and also akin to Old English magan (“to be able to”). See also may.
Etymology from Wiktionary, CC BY-SA