半径

1. Origins, structure, and basic meaning

• Structure: 半径 is read as はんけい and is a noun for a radius, the segment or distance from the center of a circle or sphere to its edge.
• Basic usage: A geometric measurement appears in 半径三センチ, while a geographic range appears in 半径一キロ.

2. Common pitfalls and similar expressions

• Difference from 直径
  • 半径 runs from the center to the edge; 直径 spans the full width through the center.
• Shape and range
  • It applies both to geometric figures and to real-world areas within a stated distance of a center point.