plejmulto

Take it apart and the meaning is exact. Plej is the superlative particle, most. Multa is much or many, and multo is the noun of it, a quantity or an amount. Plejmulto is therefore the greatest amount: the largest part of a set, the bulk of it, the vast majority. There is no irregularity anywhere in the build, and once you have it you also have plejparto, most of it, and plejofte, most often, which work identically.

Plejmulto against plimulto, which is the thing to get right. Pli is the comparative, more. So plimulto is the greater of two parts: more than half, a majority in the counting sense. That is the word for votes, seats and decisions, and it is the one you want in absoluta plimulto, an absolute majority, or per plimulto de voĉoj, by a majority of votes. Plejmulto is the loose everyday one, the great bulk, most of them, with no arithmetic implied. Plenty of speakers blur the two and dictionaries record both usages, but the distinction is real, it is available for free from the two particles, and using it makes you sound like somebody who knows the language rather than somebody translating from English majority.

The everyday alternatives. Very often the neatest thing is not this noun at all: la plej multaj homoj is most people, plejparte is mostly, preskaŭ ĉiuj is nearly all. Use plejmulto when you want the group as a single lump you can put a verb on. Its opposite is malplimulto, a minority, which reverses plimulto rather than this word, because a minority is defined against the counted majority.