What is Plural?

Legal Definition
The plural (abbreviated PL), in many languages, is one of the values of the grammatical category of number. Plural of nouns typically denote a quantity other than the default quantity represented by a noun, which is generally one (the form that represents this default quantity is said to be of singular number). Most commonly, therefore, plurals are used to denote two or more of something, although they may also denote more than fractional, zero or negative amounts. An example of a plural is the English word cats, which corresponds to the singular cat.

Words of other types, such as verbs, adjectives and pronouns, also frequently have distinct plural forms, which are used in agreement with the number of their associated nouns.

Some languages also have a dual (denoting exactly two of something) or other systems of number categories. However, in English and many other languages, singular and plural are the only grammatical numbers, except for possible remnants of the dual in pronouns such as both and either.
-- Wikipedia
Legal Definition
A term used in grammar, which signifies more than one.

2. Sometimes, however, it may be so expressed that it means only one, as, if a man were to devise to another all he was worth, if he, the testator, died without children, and he died leaving one child, the devise would not take effect. See Dig. 50, 16, 148; Id. 35, 1, 101, 1; Id. 3 1, 17, 4 Code, 6, 49, 6, 2; Shelf. on L 559, 589. See Singular.
-- Bouviers Law Dictionary
Legal Definition
Containing more than one; consisting of or designating two or morn. Webster.
-- Black's Law Dictionary