  Since the 1960s, the question whether markets are efficient or not is
controversially discussed. One reason for the difficulty to overcome the
controversy is the lack of a universal, but also precise, quantitative
definition of efficiency that is able to graduate between different states of
efficiency. The main purpose of this article is to fill this gap by developing
a measure for the efficiency of markets that fulfill all the stated
requirements. It is shown that the new definition of efficiency, based on
informational-entropy, is equivalent to the two most used definitions of
efficiency from Fama and Jensen. The new measure therefore enables steps to
settle the dispute over the state of efficiency in markets. Moreover, it is
shown that inefficiency in a market can either arise from the possibility to
use information to predict an event with higher than chance level, or can
emerge from wrong pricing/ quotes that do not reflect the right probabilities
of possible events. Finally, the calculation of efficiency is demonstrated on a
simple game (of coin tossing), to show how one could exactly quantify the
efficiency in any market-like system, if all probabilities are known.
