第一線

1. Origins, structure, and basic meaning

• Structure: 第一線, read だいいっせん, is the forefront or front line of active work in a field.
• Basic usage: 研究の第一線で働く places someone at research's forefront; 第一線を退く describes stepping back from active leadership.

2. Common pitfalls and similar expressions

• Working at and retiring from the forefront
  • The research context is figurative, not a battle line; 退く marks the sphere left with を.