A plural tantum is a noun that only occurs in the plural and does not have a singular counterpart. It ends with a plural affix such as the "s" in trousers and scissors. Tantum is a Latin word which means "only" and plurale is Latin for "plural," so the literal translation of the term is "plural only." It is often referred to as "plurale tantum."
There are many languages which have pluralia tantum such as English, Russian, Swedish and Dutch, but a plural tantum in one language is not necessarily the same plural tantum in another. Scissors, for example, is a plural tantum in English but not in Swedish where the word "en sax" means "a scissor." The assignation of pluralia tantum seems to be an arbitrary one. Pluralia tantum are usually countable nouns but with the aid of the word "pair" as in three pairs of scissors.
Many, but not all, pluralia tantum denote objects made up of two parts such as trousers or goggles. Similarly, not all nouns which represent objects made up of two sides take this form. Bicycle, for example, is a regular noun with both a plural and a singular.
The opposite of a plural tantum is singulare tantum which means the noun only appears in singular form. These are usually non-countable nouns such as "information" and "dust." These nouns fall into three categories: mass nouns, like wood and air; abstract nouns such as anger and hate and collective nouns such as fruit and rice.
Plural tantum nouns are relatively rare in comparison to their more regular counterparts. Sometimes they appear in compound nouns as themselves such as "arms race" or "jeans maker" but they can also appear as singulars in compounds like "trouser pocket." There does not seem to be any rhyme or reason in their usage and this can cause difficulty for the learner of a second language. Pluralia tantum are exceptions to the rule and so their usage must be learned by heart. For the native speaker, their use may seem logical but in a foreign language, they can be very confusing.
With the use of words such as scissors or trousers, there is no immediate conception of number. For example, with the command "give me those scissors," there could be one pair or many pairs of scissors that should be handed over. Also, it is not correct to use the referring pronoun "it" instead of trousers. "They" should be used even if there is only one pair of trousers being referred to. These are grammatical constructions in which the number of the word being used does not regularly agree with it; rather there is an implication of the number. This is what is called in grammatical terms, synesis.