srew-mn̥-
“a flow, stream, current”stream, rheum, rhythm, catarrh
Nominal of *srew- giving English stream, Greek rheuma > rheum, rhythmos > rhythm, catarrh.
Discussion
The PIE nominal form *srew-mn̥- (a flow, a stream, a current) is built from the verbal root *srew- (to flow) with the instrumental-result suffix *-mn̥-, a derivational pattern that produced nouns denoting the product or instrument of an action. The literal meaning is "that which flows" or "a flowing" — stream as process rather than place.
The most recognisable reflex is English stream itself, from Old English strēam, from Proto-Germanic *straumaz, which continues *srew-mn̥- with regular Germanic sound changes: PIE *sr- > PGmc *str- (epenthetic *t after *s before *r is a characteristic Germanic development). The cognate set is wide: German Strom (river, current, electrical current), Dutch stroom, Old Norse straumr (current, stream), and Swedish/Danish strøm all preserve the same form.
Greek ῥεῦμα (rheûma, "stream, flow, current") is the transparent Greek reflex, from *srew-mn̥- with the regular Greek loss of initial *s- before *r- (the so-called s-mobile or, in this case, regular Greek initial simplification). Rheûma gave English rheumatism (originally "a flowing of bodily humours," reflecting the Hippocratic theory that disease was caused by excess fluid flowing to a joint), rheum (watery discharge), and the combining form rheo- (flow), as seen in rheology (the study of flow) and diarrhoea (literally "flowing through").
The bare root *srew- without the *-mn̥- suffix also produced significant descendants. Sanskrit sravati ("it flows") and the related noun srotas ("stream, current") continue the root in the Indo-Iranian branch. Old Irish sruaim ("stream") provides the Celtic reflex. Old Church Slavonic struja ("stream") and Russian strujá ("jet, stream") show the Balto-Slavic continuation.
The suffix *-mn̥- is itself of theoretical interest. It is one of several PIE suffixes forming result nouns from verbal roots: compare *ǵenh₁-mn̥- ("birth, offspring," from *ǵenh₁- "to beget," giving Latin germen and English germ), and *sed-mn̥- ("seat, sitting place," from *sed- "to sit"). The productivity of this pattern — verb + *-mn̥- = thing produced by the action — is well established in PIE morphology and survives in English -ment (from Latin -mentum, itself from *-mn̥-to-), visible in words like statement, movement, and instrument.
The semantic range of *srew- descendants extends beyond water. German Strom means "electrical current" as well as "river," a metaphorical extension that mirrors the English double sense of current (water flow and electrical flow). The conceptual equation of flowing water with flowing energy is attested in multiple IE branches, suggesting either a very old metaphor or an independent parallel development driven by the physics of the analogy.