"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 often words which denote objects that occur or function as pairs or sets, such as spectacles, trousers, pants, scissors [...]."
Scissors is an example of a plurale tantum, or an English word that only has a plural form that represents a singular object. (Plurale tantum is not a plurale tantum: its plural is pluralia tantum). Though pluralia tantum name single objects, they are grammatically plural: "the scissors are on the table," "my pants are in the dryer." The shenanigans of English are myriad.
- He was wearing trousers this morning when he arrived, now he's wearing shorts.
- He was wearing a pair of trousers this morning when he arrived.
Nouns like scissors,pants, trousers, etc. can be used as is in the singular.
- Pass me those scissors that are on the table, please.
is the same as:
- Pass me that pair of scissors that is on the table, please.
However, in the plural, we must add the word two or three to the word pairs, etc.
- There were five pairs of shorts hanging from the clothes line.
- She had three pairs of glasses in her bag.