The article analyses the least negotiable group of the synonymy of terms – heterogeneous synonyms, i.e. the synonymy of Lithuanian terms and loanwords used in the field of computer science. Specifically, the focus is on the synonymy characteristic of the primary stages of terminology subsystems formation. The aim of the research is to examine the structure of heterogeneous synonymous terms and to analyse the changes of the synonymy in the computer science. In terms of origins, two groups of synonymous terms emerge in the “Encyclopaedic Dictionary of Computer Science”: those of single origins, i.e. purely Lithuanian or exclusively international synonymous terms, and those of various origins, i.e. Lithuanian and international synonymous terms...