The word "serious" can have several nuanced meanings depending on context. Here's a formal and concise definition:

Serious (adj.):
Characterized by deep thought, importance, or earnest intent; not joking, superficial, or trivial.

In specific contexts:
Medical: A "serious condition" implies significant risk, potentially life-threatening but not necessarily critical.

Legal: A "serious offense" refers to one that carries substantial penalties or consequences.

Everyday usage:

"Are you serious?" – meaning, Are you being sincere or not joking?

"This is a serious problem." – meaning, This issue demands urgent attention and has significant implications.