Unique user is an individual IP address as measured by Google (It’s not us doing any measuring). The Google report states ‘1,007,195 people visited this site…’. But I don’t say that. All that can be measured is IP addresses or in basic terms, individual computers*
* I know it’s not quite as simple as that and there’s all sorts of things that alter the numbers, like 1 IP = lots of people on a network at work or IP addresses changing each time some users login etc… But as far as the internet is concerned a Unique user is at least a reasonably standard measure that can be used to make comparisons between websites. So it’s as close an approximation to a measure of people as is generally available.