From mega- +‎ year.

megayear (plural megayears) (abbreviated as: Myr)

1. A million years, a thousand thousand years.
   • 1984, J. B. Cadwallader-Cohen, W. W. Zysiczk, R. B. Donnelly, “The chaostron: an important advance in learning machines”, in Communications of the ACM, 27(4), p.357:

     After three hours the machine had not printed its response to the first input pattern; evidently the rate of learning under these conditions is very low (we judge it to be on the order of 10^-6 concepts for megayear).

   • 1997, Ian Lerche, Geological Risk and Uncertainty in Oil Exploration, page 231:

     We know from isotopic measurements, stereochemistry, and gas chromatography/mass spectrometry, that the production of hydrocarbons in the Earth occurs on timescales ranging from fractions of a megayear to hundreds of megayears, in a temperature regime in the rough range of 0-200°C, and in an evolving environmental "smog" of sedimentary pores containing water laced with varying amounts of ions, molecules, and minerals.

   • 2001, Fred Howard, Taming the Vipers in the Slough, page 400:

     As to that time span, all clearly definite forms of life or species seem to take a megayear more or less to come about and be present for a while.

• mega-annum
• millionennium

• kiloyear
• gigayear, gigannum