APA Dictionary of Psychology

n. the number of times that a given value (the base) has to be multiplied by itself to produce a specific quantity. For example, the logarithm for the quantity 81 and the base 3 is the number 4 (3 x 3 x 3 x 3 = 81). Logarithms are given by the generic formula y = log_ax, thus 4 = log₃81.