Tobin's q
Tobin's q (also known as q ratio and Kaldor's v) is the ratio between a physical asset's market value and its replacement value. It was first introduced by Nicholas Kaldor in 1966 in his article "Marginal Productivity and the Macro-Economic Theories of Distribution: Comment on Samuelson and Modigliani". It was popularised a decade later, however, by James Tobin, who describes its two quantities (1977):

One, the numerator, is the market valuation: the going price in the market for exchanging existing assets. The other, the denominator, is the replacement or reproduction cost: the price in the market for newly produced commodities.