A series of indexes that tracks the financial performance of different types of mutual funds. Lipper, which is owned by Reuters, allows investors to benchmark the performance of a mutual fund investment against an index of 30 funds that also belong in that investment category. Lipper creates indexes for each major investment style, such as small- or large-cap funds, value funds and growth funds. Much like the S&P 500 allows stock investors to compare their returns to a market benchmark, a Lipper index aggregates the 30 biggest funds of a given investment style. For example, an investor could compare the performance of a small-cap mutual fund against the Lipper Small-Cap Index, which would combine the 30 biggest small-cap funds, based on asset size.