半径
はんけい · hankei · JLPT N2 · noun
radius
⭕ 半 (half) + 径 (diameter, path) = half of a diameter = radius
径 means diameter or path, and 半 means half. A radius runs from the center to the edge, which is half of a diameter, so the compound fits the meaning exactly.
半 — 八 (split) + 牛 (ox/cow)
半 originally depicted an ox (牛) being divided in two, shown by the splitting strokes 八 on top. The image of cleaving an ox down the middle gave rise to the meaning 'half' or 'to divide into halves'. The modern sense of 'half' or 'partial' comes directly from this ancient act of splitting.