This is the name of your server. If your logfiles are from the domain "mydomain.com", usually the server name is "www.mydomain.com". Syntax is something like : <host>.<domain>.<country>.
Use the right name as the field is used to extract domain name and can be used to exclude domain stats.
Avoid to use just a localhost name as you won't be able to compute domain stats later.

Value (ex) Description OK
www.mydomain.com Domain will be "mydomain.com" Yes
www.subdomain.mydomain.com Domain will still be "mydomain.com" Yes
myhost.mydomain.com Still the same value for domain Yes
ftp.mydomain.com if using a ftp server Yes
localhost No way to extract domain name. Domain filtering will be not available. No
127.0.0.1 No way to extract domain name. Domain filtering will be not available. Yes