Unique means without a like or equal; unmatched; unequaled; single in its kind; uncommon or rare. It cannot logically be used in a comparative or superlative for. Something may be more or less rare but NOT more or less unique.
radically distinctive and without equal; "he is alone in the field of microbiology"; "this theory is altogether alone in its penetration of the problem"; "Bach was unique in his handling of counterpoint"; "craftsmen whose skill is unequaled"; "unparalleled athletic ability"; "a breakdown of law unparalleled in our history"
A unique visit/view/etc. is a instance that only occurs the first time that event transpires during the report period. For example: If Joe were to visit a website on Monday and on Wednesday, his visit would show up as a unique visit, for both days, when a daily report is run.
"There can not be more than one equivalent item of this type within the scope indicated. We may say that customer numbers must be unique (within our organization or database) or we may say that there can only be one successor on a relationship, meaning there can't be more than one of that type in that set."
user Unique individual who has either accessed a site or been served unique content and/or ads such as e-mail, newsletters, interstitials and pop-up ads. Equivalent to a site's reach. Unique users can be identified by user registration or cookies.