In astronomy, geology, and paleontology, mya is an abbreviation meaning "million years ago". It denotes a time measured from the present into the past; numerically, one mya equals 106 years before the present. The phrase "before present" (B.P. or BP) is conventionally anchored to the year 1950.

Meaning and conversion

When used in statements of age, mya indicates how many millions of years have passed since an event. For example, 1 mya = 1,000,000 years ago and 10 mya = 10,000,000 years ago. Writers sometimes express the same concept with scientific notation (e.g., 1 × 106 years ago) or with other standardized units.

Notation and preferred forms

The lowercase form "mya" has been widely used in popular and older scientific texts. In formal scientific contexts, authors often prefer the Latin-derived term megaannum, abbreviated Ma, which aligns with internationally recommended notation for geological time. A related abbreviation, Myr, is sometimes used to indicate a span of one million years rather than a point in time measured "ago".

Usage notes

  • Use "mya" to state how many millions of years before the present an event occurred (for example, "about 66 mya").
  • Use "Ma" in technical writing to label an absolute age (for example, "66 Ma" for an age of 66 million years) and consider "Myr" when describing durations (for example, "a 5 Myr interval").
  • Be aware that some journals and professional bodies recommend Ma and ka (kiloannum) over mya and ka for consistency with scientific conventions.

The abbreviation "mya" itself is an acronym formed from the initial letters of the words "million," "years," and "ago." Authors choosing unit notation should follow the style guide appropriate to their discipline or publication.