Quantity and Variable

noun

• How much there is or how many there are of something that you can quantify.
  • synonyms: measure, amount
• An adequate or large amount.
  • usage: "he had a quantity of ammunition"
• The concept that something has a magnitude and can be represented in mathematical expressions by a constant or a variable. .

• (n.) The attribute of being so much, and not more or less; the property of being measurable or capable of increase and decrease, multiplication and division; greatness; and more concretely, that which answers the question "How much?"; measure in regard to bulk or amount; determinate or comparative dimensions; measure; amount; bulk; extent; size.
• (n.) The extent or extension of a general conception, that is, the number of species or individuals to which it may be applied; also, its content or comprehension, that is, the number of its constituent qualities, attributes or relations.
• (n.) That which can be increased, diminished or measured; especially (Math.), anything to which mathematical processes are applicable.
• (v.t.) To modify or qualify with respect to quantity; to fix or express the quantity of; to rate.
• (n.) The relative duration of a tone.
• (n.) A determinate or estimated amount; a sum or bulk; a certain portion or part; sometimes, a considerable amount; a large portion, bulk or sum; as, a medicine taken in quantities, that is, in large quantities.
• (n.) The measure of a syllable; that which determines the time in which it is pronounced; as, the long or short quantity of a vowel or syllable.

noun

• Something that is likely to vary; something that is subject to variation.
  • usage: "the weather is one variable to be considered"
• A quantity that can assume any of a set of values.
  • synonyms: variable quantity
• A star that varies noticeably in brightness.
  • synonyms: variable star
• A symbol (like x or y) that is used in mathematical or logical expressions to represent a variable quantity.

adjective

• Liable to or capable of change.
  • usage: "rainfall in the tropics is notoriously variable"; "variable winds"; "variable expenses"
• Marked by diversity or difference.
  • usage: "the varying angles of roof slope"; "nature is infinitely variable"
  • synonyms: varying
• Used of a device) designed so that a property (as e. G. Light) can be varied. (
  • usage: "a variable capacitor"; "variable filters in front of the mercury xenon lights"

• (n.) Those parts of the sea where a steady wind is not expected, especially the parts between the trade-wind belts.
• (n.) That which is variable; that which varies or is subject to change.
• (a.) Liable to vary; too susceptible of change; mutable; fickle; unsteady; inconstant; as, the affections of men are variable; passions are variable.
• (n.) A quantity which may increase or decrease; a quantity which admits of an infinite number of values in the same expression; a variable quantity; as, in the equation x2 - y2 = R2, x and y are variables.
• (a.) Having the capacity of varying or changing; capable of alternation in any manner; changeable; as, variable winds or seasons; a variable quantity.
• (n.) A shifting wind or one that varies in force.

Variable is an element in a model. For example, in the model RS&Pt+1 = a + b Tbill t + et, where RS&Pt+1 is the return on the S&P in month t+1 and Tbill is the Tbill return at month t, both RS&P and Tbill are "variables" because they change through time; i. E., they are not constant.