This article is about the unit of time. For the Magic: the Gathering creatures, see Creatures of Magic: The Gathering.

The symbol myr was formerly used in English-language geology and astronomy as a unit of one million years. It is an abbreviation for 'million years' and lower case is usually used.

In English-language technical literature in these fields, the term 'Ma' is preferred, as this conforms to ISO 31-1 and NIST 811 recommended practices. "M" denotes mega- (10⁶), and "a" denotes years (from the Latin annum. Note that "ma" would technically imply a milliannum (also called a milliyear in English usage), a period of a thousandth of a year or about 8 hours.

The correct ISO 31-1 usage is megaannum or Ma which unambiguously denotes a duration of 10⁶ years. To denote a date one would add ago or bp to denote before present.

In non-SI usage, Ma was used to denote a specific number of millions of years ago, but it was not properly used to describe a duration, 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.

See also

• Megaannum
• Byr
• kyr