Pluralia tantum

A plurale tantum (Latin for "plural only", plural form: pluralia tantum) is a noun that appears only in the plural form and does not have a singular variant for referring to a single object. In a less strict usage of the term, it can also refer to nouns whose singular form is rarely used.

In English pluralia tantum are frequently words which denote objects that occur or function as pairs or sets, such as spectacles, trousers, pants, scissors, clothes, electronics, bagpipes, or genitals. However, this is not the case for all languages. In some languages they refer to points or periods of time (for example, Latin kalendae "calends, the first day of the month", German Ferien "vacation, holiday") or to events (for example, Finnish häät "wedding"). In some cases there is no obvious semantic reason for a particular noun to be plurale tantum. Hebrew mayim "water", Dutch hersenen "brain", Swedish pengar and Russian den'gi [деньги] "money" are pluralia tantum.

A bilingual example is the Latin word fasces, which was brought into English; when referring to the symbol of authority, it is a plurale tantum noun in both languages.

In English, some plurale tantum nouns have a singular form, used only attributively. Phrases such as "trouser presses" and "scissor kick" contain the singular form, but it is considered nonstandard to say "a trouser" or "a scissor" on their own. That accords with the strong preference for singular nouns in attributive positions in English, but some words are used in the plural form even as attributive nouns (such as "clothes peg", "glasses case").

In English, a word may have definitions which are plurale tantum. The word "glasses" (a set of corrective lenses to improve eyesight) is plurale tantum. In contrast, the word "glass" (either a container for drinks, a count noun, or a vitreous substance, a mass noun) may be singular or plural. Some words, such as "brain" and "intestine", can be used as either plurale tantum nouns or count nouns.

...
Wikipedia