What is onus in law?

Could you please clarify for me what exactly is meant by the term 'onus' in the context of law? Is it a legal responsibility or burden that a party is required to bear, or does it have a more nuanced meaning? Additionally, how does the concept of onus differ from other related legal terms such as liability or obligation? I'm eager to understand how it's applied in various legal scenarios and how it impacts the outcomes of cases.