"Double" has a number of meanings, the most basic of which is "a two-fold nature," and the word may be used as an adjective, verb or noun. As an adjective, the word may also be used to denote relative quantity or quality. As a verb, it can refer to the act of increasing such a relative quality or quantity.
As a noun, "double" may refer to something's duplicate.
The word entered English from the Old French word "doble," meaning "double," "two-fold" or even "deceitful." The Old French word, in turn, derives from the Latin "duplus," meaning "twice as much," from "duo" ("two") and "plus" ("more"). Each of these meanings is retained by modern English usage.