There's a third explanation of "Яndex" being "языковой индекс" i.e. "language-aware index". Russian language have complicated morphology with three genders and

There's a third explanation of "Яndex" being "языковой индекс" i.e. "language-aware index". Russian language have complicated morphology with three genders and six grammatical cases, somewhat similar to Latin. Searching by an exact word-match almost never gives good results, and neither Yahoo nor AltaVista could offer any better in 1997 -- hence Yandex was built.