diacritic, in the proper sense, is the acute accent ⟨◌́⟩ (Russian: знак ударения 'mark of stress'), which marks stress on a vowel, as it is done in Spanish...

Language of the Russian Academy of Sciences, an optional acute accent (знак ударения) may, and sometimes should, be used to mark stress. For example, it...

S2CID 144935892. Лопатин, Владимир Владимирович, ed. (2009). § 116. Знак ударения. Правила русской орфографии и пунктуации. Полный академический справочник...