Myr
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Myr is often used in geology and astronomy as a unit of one million years. The "M" is the SI prefix for Mega or million, with "yr" simply an abbreviation for "year".
Often the "M" is shown in lowercase, as in "100 myr", which is an incorrect usage, because a lowercase "m" would signify milli, or thousandths, such that "100 myr" would indicate a period of just over a month. "Myr" itself is often considered incorrect, with some preferring to use "My", as "y" is considered the proper abbreviation for year.
Related to Myr is "Ma", or Mega-annum. Ma is used to denote a specific number of millions of years ago, but it is not (should not be) used to describe a duration of time, so: the Cretaceous started 145 Ma and ended 65 Ma, but it lasted for 80 Myr (or 80 My). More often, the term "Mya" (million years ago) is used in these contexts.