A benchmark index is covered in full in the entry Benchmark (Investing), which explains how benchmarks are chosen, how funds disclose them and where they fall short. This page gives a short definition and explains the two names.
A benchmark index is a market index used as a yardstick against which the return and risk of a fund, portfolio or manager are measured. Familiar examples include the S&P 500 for large US shares, broad global equity indices and bond indices for fixed income funds. A fund that tracks such an index closely, as many exchange-traded funds do, aims for a small tracking error, while an actively managed fund tries to beat it after fees.
The two names overlap because most benchmarks are indices, but not all are. A benchmark can also be a peer group of similar funds, a blended mix of indices or an absolute target such as inflation plus a margin. "Benchmark index" is the narrower phrase, used when the yardstick is a published index. In crypto, providers publish benchmark indices and reference rates for bitcoin and other assets that are used to price funds and futures.