Tobin's q
James Tobin (1918-2002) used the letter q to denote the ratio of a firm's market value (that is, something like the price the entire firm would fetch on the stock market at a given moment) to a firm's book value (something like the cost to replicate all of the firm's capital stock). If q is greater than one, further investment in capital by the firm will be profitable; if q is less than one, further investment in capital by the firm will not be profitable.
Marginal q: The concept
The q measure was quickly adopted by a variety of different fields within economics, including macroeconomics, finance, and the study of investment. However, economists quickly realized that Tobin's q really applied to the marginal dollar of investment. Although a firm's average q was a useful statistic in its own right, the firm's investment decisions would be made at the margin. Hence the need for marginal q.
Marginal q: The website
When we were discussing creating this web site, we wanted to avoid cumbersome URLs; for example, those with hyphens, or those with unusual domains. However, we found that almost all of the catchy, easy-to-remember domain names had already been taken. Thus, we looked for a term of art in economics that would be familiar to economists but which would not have already been grabbed by someone else. Because every economist has at least a passing familiarity with Tobin's q in general, and marginal q in particular, this term won out over the other contenders.
Further Reading: