秘密

1. Origins, structure, and basic meaning

• Structure: 秘密（ひみつ） means something unknown to others or kept from them.
• Basic usage: 秘密にしておく means keeping something secret; 秘密を語る means telling its contents.

2. Common pitfalls and similar expressions

• A secret and keeping something quiet
  • 秘密 can name the hidden information itself, while 内緒 often emphasizes keeping something from others.
• Who is kept uninformed
  • In 本人には秘密にしておこう, the plan is specifically being kept from the person whose birthday is being celebrated.