Quoting from the Scrum Guide :

When the Product Backlog item or an Increment is described as "Done", everyone must understand what “Done” means. Although this varies significantly per Scrum Team, members must have a shared understanding of what it means for work to be complete, to ensure transparency. This is the "Definition of Done" for the Scrum Team and is used to assess when work is complete on the product Increment.

A team's shared definition of done is one of the most overlooked aspects of scrum. It's important that the whole team agree on a set of criteria that they will work to. It should be a living document that can be reviewed and modified during scrum's "inspect and adapt" cycles. Usually a team new to scrum will create a basic definition and add to it as they mature.

Here's an example that I often use as a starting point for discussion:

• All production code must have unit tests that pass.
• All production code should be written by a pair (or)
• All production code must reviewed by another developer.
• All production code must be checked into source control.
• Documentation is written for the feature.
• CI build passes.
• Application is deployed from build server not local builds
• Application deploys without errors.
• System tests pass.
• Product owner sign off.