転回
てんかい · tenkai · JLPT N1 · noun
revolution
🎡 転 turn + 回 round = てんかい: a turn all the way around; revolution, rotation
Both characters are about turning: 転 means roll or turn over, and 回 means go around or occurrence. 転回 stresses the turning itself, while 回転 is the everyday word for spinning.
回 — enclosure 囗 + smaller 囗 inside
回 is a pictograph of a swirling whirlpool or coiled spiral, shown as nested squares in modern form. The original meaning was 'to revolve' or 'turn around,' which extended to 'return' and 'a time/occurrence.' It also appears as a phonetic in characters like 茴 and 廻.