mega-

A decimal multiplier prefix in SI, meaning 1,000,000. It has also been attached to many non-SI units, where in a few cases it does not mean 1,000,000—for example, megabyte.

Use of the mega- (and micro-) prefixes was recommended by the British Assn. Committee for the Selection and Nomenclature of Dynamical and Electrical Units in its influential Report of 1873, the one that established the cgs system as the preferred system for scientific work.

British Association for the Advancement of Science.
Reports of the Committee on Electrical Standards appointed by the British Association....
London: E. and F. N. Spon, 1873.

Examples

In order to express multiples and sub-multiples the prefixes mega or micro are used, which are respectively a million times as great or as small. Thus a megohm is 10⁶ ohms, that is, 10¹⁵ absolute units of resistance. In like manner a microhm is 10^-6 ohm, that is, 10³ = 1000 such units.

E. Atkinson.
Elementary Treatise on Physics Experimental and Applied for the use of colleges and schools. Translated and edited from Ganot's Éléments de Physique. Twelfth edition.
London: Longmans, Green, and Co., 1886.
Page 939.