ETYM Latin pluralis, from plus, pluris, more; cf. French pluriel, Old Fren. plurel. Related to Plus.
(Homonym: pleural).
Grammatical number category referring to two or more items or units.

The form of a word that is used to denote more than one; SYN. plural form.
Indication of number. Most English words form their plurals by the addition of s, as in boy, boys; cat, cats; book, books. The ending es is the next most common, as in watch, watches; church, churches; gas, gases; princess, princesses; bush, bushes; tax, taxes. This is simply to aid pronunciation.
There are spelling rules that aid the forming of plurals. The f/ves pattern seems clear enough in these words, for instance: wolf, wolves; half, halves, shelf, shelves; life, lives; wife, wives; knife, knives. But, as in the case of most English spelling rules, there are a disappointing number of exceptions: chief, chiefs; roof, roofs, for example. The “change the y into i and add es” rule works better: spy, spies; baby, babies; lady, ladies.
Here the only exceptions are those where a vowel precedes the final y, as in donkey, donkeys; vacation, vacations.
The plurals that cause most trouble for young spellers are those that follow no pattern—children, geese, women —and those that are the same in their singular and plural forms: deer, for example.