Tobin's q was developed by James Tobin (Tobin 1969) as the ratio between the market value and replacement value of the same physical asset:
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 the newly produced commodities. We believe that this ratio has considerable macroeconomic significance and usefulness, as the nexus between financial markets and markets for goods and services.
Read more about Tobin's Q: Application, Other Influences On Q, Tobin's Marginal Q, P/B Ratio, Criticism