Definition of "megayear"

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).

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

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.

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

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.

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