The “original” cyrillic, whose encoding was adopted by the LaTeX project as OT2. The fonts are distributed as part of the AMS fonts collection (both as Metafont source and as Adobe Type 1).

Note that OT2 is nowhere used in hyphenation tables, so that, for fully satisfactory typesetting of cyrillic languages, the LH font set, or an Unicode font that offers cyrillic glyphs, is necessary. The LH fonts are also available in Adobe Type 1 format as part of the CM-super bundle)

The author is The American Mathematical Society.