交流
こうりゅう · kouryuu · JLPT N2 · noun
exchange
交 (exchange, hand over) + 流 (flow, current): things flowing back and forth, and electricity that flows back and forth
交 means to hand over or cross; 流 means to flow. Exchange is flow passing between two sides. Alternating current flips direction back and forth, so the same two characters fit.
交 — top 亠 + 父-like crossed legs
Originally a pictograph of a person standing with legs crossed, showing the idea of crossing or intersecting. From this concrete image, the meaning extended to abstract senses like 'exchange,' 'interact,' and 'hand over.' Modern uses such as 交流 (exchange) and 交通 (traffic) all trace back to this core notion of crossing.
流 — water radical 氵 + 㐬 (phonetic, child being born)
流 is a semantic-phonetic compound. The water radical 氵 gives the meaning, while 㐬 depicts an infant being born head-down with amniotic fluid, suggesting downward flow. Together they convey water flowing, and by extension 'to flow' or 'current'.