What is Arbitrage Pricing Theory?

Legal Definition
In finance, arbitrage pricing theory (APT) is a general theory of asset pricing that holds that the expected return of a financial asset can be modeled as a linear function of various macro-economic factors or theoretical market indices, where sensitivity to changes in each factor is represented by a factor-specific beta coefficient. The model-derived rate of return will then be used to price the asset correctly—the asset price should equal the expected end of period price discounted at the rate implied by the model. If the price diverges, arbitrage should bring it back into line.

The theory was proposed by the economist Stephen Ross in 1976.
-- Wikipedia
Legal Definition
A financial theory posited as a testable, and more flexible, alternative to the CAPITAL ASSET PRICING MODEL (CAPM), based on the concept that multiple linear RISK factors influence the return of a security, and the factors can be estimated through principal components/factor analysis. By understanding the risk and return contribution of each factor, an optimal portfolio can be created. APT, like CAPM, makes use of BETA as a measure of risk. The singlefactor APT return is given by: where E(rj) is the expected return of security j, E(rf) is the expected RISKFREE RATE, 1 is the slope of risk factor 1, and 1 is the beta related to risk factor 1 and security j. The equation can be expanded to multifactor form, with z risk factors: Additional references: Roll (1977), Roll and Ross (1980), Ross (1976).