直径
ちょっけい · chokkei · JLPT N2 · noun
diameter
直 (straight) + 径 (path/diameter): a straight line passing through the center, from edge to edge
直 means straight and 径 means path or passage. A diameter is the straight line that crosses a circle through its center.
直 — 十 ten + 目 eye + 乚 hidden line
Originally depicted an eye (目) looking straight along a vertical line, suggesting a direct, unswerving gaze. The form combines 目 'eye' with strokes indicating straightness. From this came the meanings 'straight,' 'upright,' and by extension 'honest' or 'direct.'