The word sobornost’ is derived from the word used in the Slavonic version of the creed to translate katholikos, ‘catholic’. It appears that some of the older texts in the Slavonic Creed simply transliterated katholikos as katholichesky, as did the Latin version and virtually all European versions, but in (or maybe by) the fifteenth century katholichesky was replaced by soborny.
It is often said that soborny is derived from the word for a council in Slavonic, sobor, but I suspect the truth is more interesting. In replacing katholichesky, the Slavonic translators went back to the root meaning of katholikos, which is formed from the Greek kath’ holon, ‘according to the whole’, and took the word to mean something like ‘taken as a whole’, ‘gathered together’. So they used the word soborny, an adjective derived from the word sobrat’, ‘to gather together’. The word for council or synod, translating the Greek synodos, meaning a ‘coming together’, a ‘gathering’ and hence ‘council’, is sobor, so the use of soborny in the creed suggested that it is in a council that the Church manifests its nature.
In a remarkable way, then, the word soborny makes a link between the Church as catholic and the Church as conciliar: between the Church as proclaiming a truth that concerns everyone, and the Church as constituted by being gathered together by God.