Definition of year over year in US English:

year over year

adverb

  • 1Repeatedly over a number of years; for years without change. Compare "year on year". Now rare.

  • 2With reference to trends, gradual increases or decreases, etc.: with or in each passing or successive year.

  • 3With reference to figures, financial data, etc.: as compared with the corresponding information from the previous year.

adjective

  • Economics and Finance (chiefly North American). Relating to or shown by a comparison of corresponding figures from two consecutive years.

Origin

Early 19th century; earliest use found in Arthur Aikin (1773–1854), natural scientist and author. From year + over + year.