Seriousness or earnest intent; being earnest or genuine about something (often used in expressions like 'to be serious about').
本気で日本語を勉強するなら、毎日練習する必要がある。
If you're serious about studying Japanese, you need to practice every day.
冗談だと思っていたが、彼は本気だった。
I thought he was joking, but he was serious.
本気の勝負を見せてくれ。
Show me a real, serious match.