In the context of software or information modeling, a happy path (sometimes called happy flow) is a default scenario featuring no exceptional or error conditions.
[1][2] For example, the happy path for a function validating credit card numbers would be where none of the validation rules raise an error, thus letting execution continue successfully to the end, generating a positive response.
Process steps for a happy path are also used in the context of a use case.
[4] Happy path testing can show that a system meets its functional requirements but it doesn't guarantee a graceful handling of error conditions or aid in finding hidden bugs.
And in contrast to a single happy path, there are a lot of different ways in which things can go wrong, so there is no single criterion to determine 'the unhappy path'.