In a Cisco Router What is the ip domain lookup command used for?

Enables DNS-based host name-to-address translation. This feature is enabled by default. So if you type something that appears to be a hostname the router will try to resolve it. This will happen three times, waiting for the dns request to time out. This can take a lot of time and when your trying to get things done could become annoying...

Translating "router123"...domain server (255.255.255.255)

Translating "router123"...domain server (255.255.255.255)

Translating "router123"...domain server (255.255.255.255)

To disable:

Router(config)#no ip domain lookup

In older versions of iOS the command was actaully no ip domain-lookup