In the context of software or information modeling, a happy path is a default scenario

Scenario (computing)

In computing, a scenario is a narrative describing foreseeable interactions of types of users and the system. Scenarios include information about goals, expectations, motivations, actions and reactions...

 that features no exceptional or error conditions, and comprises the sequence of activities that will be executed if everything goes as expected. For example, the "happy path" for a function that validates credit card numbers would be the one where none of the validation rules raise an error, thus letting execution continue successfully to the end, generating a positive response.

Happy path testing is a well-defined test case

Test case

A test case in software engineering is a set of conditions or variables under which a tester will determine whether an application or software system is working correctly or not. The mechanism for determining whether a software program or system has passed or failed such a test is known as a test...

that uses known input, that executes without exception and that produces an expected output.

Golden path is a synonymous term for happy path.