無効

1. Origins, structure, and basic meaning

• Structure: 無効 is read as むこう and is a noun and na-adjective for a ticket, contract, rule, or other item being invalid or without effect.
• Basic usage: Use 無効です for a present judgment and 無効となる for becoming invalid.

2. Common pitfalls and similar expressions

• The cause depends on context
  • Expiration and a missing signature are different invalidating conditions; the word itself does not specify the cause.
• Contrast with 無駄
  • 無効 means lacking validity or effect, while 無駄 means effort or expense is wasted.