What is the onus in law?

Could you elaborate on the concept of "onus in law"? I understand it refers to a legal responsibility or obligation, but how does it specifically apply in different legal contexts? For instance, in criminal law, does the onus rest with the prosecution to prove guilt beyond a reasonable doubt, or does the defendant have an onus to prove innocence? Similarly, in civil disputes, how is the onus of proof allocated, and what factors influence this determination? Additionally, are there any specific examples of cases where the onus in law has been a crucial aspect of the legal decision-making process?