What is the oAuth domain that is mentioned in the StackApps app registration form? I googled it but couldn't find a good description.
The OAuth domain name is the domain name that will be used to restrict the value of the
redirect_uri parameter when authenticating from your application. This is done as a security precaution to prevent others from using your API key and redirecting to a domain name that does not match the one specified in the registration form.
The API documentation page for authentication makes the following brief comment when describing the
"redirect_uri - must be under an apps registered domain"