The power to which a base, usually 10, must be raised to produce a given number. If nx = a, the logarithm of with as the base, is symbolically, log n a = x. For example, 103 = 1,000; therefore, log10 1,000 = 3. The kinds most often used are the common logarithm and the natural logarithm. Logarithmic spiral: A spiral curve such that radii drawn from its pole or eye at equal angles with each other are in continual proportion. See equiangular spiral.