Year Over Year - YOY

What Does Year Over Year - YOY Mean?
A method of evaluating two or more measured events that compares the results of measurement at one time period with those from another time period (or series of time periods), on an annualized basis. Year-over-year comparisons are a popular way to evaluate the performance of investments. Any measurable events that recur annually can be compared on a year-over-year basis - from annual performance, to quarterly performance, to daily performance.

Investopedia explains Year Over Year - YOY
Year-over-year performance is frequently used by investors seeking to gauge whether a company's financial performance is improving or worsening. For example, a business may report that its revenues have increased for the third quarter on a year-over-year basis for the last three years. This means that revenues at that company in the third quarter of year 3 were higher than revenues in the third quarter in year 2, which were higher than revenues in the third quarter of year 1.

As another example, a mutual fund that returned 50% last year may have a YOY return of 12%, as the year-over-year return takes into account each annual return since the fund's inception.