A visitor to a web site is just that: a visitor. Not a unique visitor.
Anyone using a web analytics package is doubtless familiar with the difference, but now it's legally official. A New York Surpeme Court judge made the ruling in favor of WebMD which has been embroiled in a $350,000 lawsuit with RDA over the subtlties of the terms.
Seems RDA had placed several insertion orders with WebMD, including one that guaranteed WebMD would send 36,000 "visitors" to an RDA-run site.
RDA insisted what the contract actually meant (as opposed to what it actually said) was "unique visitors."