What the lexicon says. συνείδησις (syneidesis) is defined first as "consciousness" and then, "In ethical sense, innate discernment, self-judging consciousness, conscience", with the note that the ethical use belongs to "Stoics and late writers". Strong's: "Co-perception, i.e. moral consciousness." It is in 30 verses.
The gap it fills, in a named source's words. The 1915 ISBE article "Heart", by J. I. Marais, states that "'heart' comes to stand for 'conscience,' for which there is no word in Hebrew", giving Job 27:6 and 1 Samuel 24:5 as examples. The Greek term does the job directly: "their conscience bearing witness therewith" (Romans 2:15, ASV).
⚠️
The limit on that observation. "No word for conscience in Hebrew" is Marais's formulation in 1915 and is a claim about vocabulary, not about whether the idea is present, the same article's examples are Hebrew texts describing exactly the experience. Later scholarship discusses the point in more detail than a 1915 encyclopedia entry does, and this file states the claim with its source and date rather than as a settled fact.
Sources: Romans 2:15; Job 27:6; 1 Samuel 24:5; TBESG entry G4893 (Abbott-Smith); Strong's Greek Dictionary (1890), entry G4893; ISBE 1915, "Heart" and "Conscience".
---