敵

1. Origins, structure, and basic meaning

• Structure: 敵 (てき) is a noun for an opposing adversary or, figuratively, a factor harmful to an ability.
• Basic usage: 強大な敵に立ち向かう identifies the adversary confronted by the protagonist, while 睡眠不足は集中力の敵だ figuratively labels sleep deprivation as harmful to concentration.

2. Common pitfalls and similar expressions

• Literal opponent and figurative hindrance
  • 敵に立ち向かう directs action toward an opponent, whereas 集中力の敵 is a nonhuman cause that degrades performance. The latter is not a literal battle with sleep deprivation.