Where does “средне-” come from?

средне- (Russian) comes from Russian сре́дний, from Old Church Slavonic срѣдьнъ, from Proto-Slavic serdьnъ, from Proto-Slavic serda, from Proto-Balto-Slavic śerd-, from Proto-Indo-European ḱerd- — heart.

Ancestry of “средне-”, step by step

StepLanguageWordMeaning
1Russianсре́днийmiddle
2Old Church Slavonicсрѣдьнъ—
3Proto-Slavicserdьnъmiddle, central
4Proto-Slavicserdamiddle; Wednesday
5Proto-Balto-Slavicśerd-—
6Proto-Indo-Europeanḱerd-heart
Every word from Proto-Indo-European ḱerd- →Every word from Proto-Balto-Slavic śerd- →Every word from Proto-Slavic serda →