Plural

Plural is a grammatical category and a language universal.

In most languages it's directly opposed to the grammatical category of 'singular' but some languages have a third category, namely 'dualis'.

To put it simply:

singular refers to 1 referent

dualis refers to 2 referents (especially for living creatures)

plural refers to more referents

Of course, that's too simple, because some singular words do refer to collections, groups, organisations and institutions [but considered as one entity: collective words] and some plural words do refer to one single referent.

In most languages the plural of words is marked in some way that makes it immediately clear to the audience that this category has been chosen.