To define a credential in TFS 2015, you can follow these steps:

1. Open Visual Studio and go to Tools > Options.
2. In the Options dialog box, expand the Team Foundation Server node.
3. Click on the Credentials tab.
4. Click on the Add button.
5. In the Add Credential dialog box, specify the following information:
   • Name: The name of the credential.
   • Description: A description of the credential.
   • Server: The name of the TFS server.
   • Username: The username for the TFS server.
   • Password: The password for the TFS server.
6. Click on the OK button.

Your credential will now be saved in Visual Studio. You can use it to connect to the TFS server without having to enter your username and password each time.

Here are some additional tips for defining credentials in TFS 2015:

• You can define multiple credentials for different TFS servers.
• You can use the same credential for multiple projects.
• You can export your credentials to a file so that you can import them into another computer.
• You can delete your credentials if you no longer need them.