Forgive the obvious answer, but it's called an, ummm, IP Address.
More specifically it's a non-RFC 1918 address:http://www.faqs.org/rfcs/rfc1918.html
Also known as a routable IP Address.
An easy way to think of it is like your house address. If you didn't have a street address how would someone else send you a letter? When you send someone else a letter, you put your return address on the envelope so they can write back to you.
Of course, if you put a false return address on the envelope, that would be like spoofing.
And if you live in an apartment, your apartment number is like an RFC 1918 Private Address, it's non-routable, but has to be translated (NAT) by the address of your apartment building. There might be hundreds of apartment buildings in the city that all use apartment numbers 1A, 1B, 1C, 2A, 2B, 2C, etc., but you are going to wrap that in the address of your apartment building.