MYA is an abbreviation of Millions of Years Ago, a common block unit of time in Generation One Transformers series. Event timelines in 2005's Transformers: The Ultimate Guide were organized in MYAs.
The backstories of the 1984 cartoon and the Dreamwave comic were both structured with major events occurring at 1 million year epochs.
Note: The MYA 'unit' is used by fans and guidebooks; it does not exist within the fiction.