Wiktionary
n. (context grammar English) A noun (in any specific sense) that has no singular form, such as scissors (in most usage).
Wikipedia
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 typically words which denote objects that function as pairs or sets (spectacles, trousers, pants, scissors, clothes, electronics, bagpipes, genitals).
Many languages have pluralia tantum, such as the Latin word kalendae, the Russian word den'gi [деньги] ("money"), the Hebrew word mayim ("water"), and the Dutch word hersenen ("brain"). 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.