Definitions of variable:

• noun:   a quantity that can assume any of a set of values
• noun:   a symbol (like x or y) that is used in mathematical or logical expressions to represent a variable quantity
• noun:   something that is likely to vary; something that is subject to variation

  Example: "The weather is one variable to be considered"

• noun:   a star that varies noticeably in brightness
• adjective:   liable to or capable of change

  Example: "Rainfall in the tropics is notoriously variable"

• adjective:   (used of a device) designed so that a property (as e.g. light) can be varied

  Example: "A variable capacitor"

• adjective:   marked by diversity or difference

  Example: "Nature is infinitely variable"