Serious; not light or trivial. Often used for grave matters or situations not to be treated jokingly (used as a na-adjective: 'serious').
その問題はシリアスに受け止める必要がある。
That problem needs to be taken seriously.
会議の雰囲気がシリアスで、冗談は言えなかった。
The mood at the meeting was serious, so I couldn't joke around.
あの映画はコメディと思っていたが、意外とシリアスだった。
I thought that movie was a comedy, but it was unexpectedly serious.