Einheitliche System Elektronischer Rechenmaschinen (German for: standardized system of electronic computers) was a treaty between the members of Comecon signed on December 23 1968 covering the development of a standardized computing system.
ESER was also the name for computers developed by this standard. Typically ESER Computers where named ЕС (which is cyrillic for "ES") followed by a four digit number (Example: EC 1834 an IBM PC XT compatible made by Robotron).
See also: History of computer hardware in communist countries