na-adjective
Describes a person, attitude, effort, or discussion that is genuinely committed and not light-hearted. Common in phrases like 真剣に考える (think seriously) and 真剣勝負 (serious contest).
彼は将来について真剣に考えている。
He is thinking seriously about his future.
She was listening to the story with a serious expression.
noun
real sword; actual blade
Refers to a genuine steel sword, as opposed to a wooden or bamboo practice weapon (木刀, 竹刀). Often used in martial arts or historical contexts.
稽古では木刀を使うが、真剣は重くて扱いが難しい。
In practice we use a wooden sword, but a real sword is heavy and difficult to handle.