Definitions of one:

• noun:   a single person or thing

  Example: "He is the best one"

• noun:   the smallest whole number or a numeral representing this number

  Example: "He has the one but will need a two and three to go with it"

• adjective:   being a single entity made by combining separate components

  Example: "Three chemicals combining into one solution"

• adjective:   used of a single unit or thing; not two or more