usually it's unique visitors but they factor in page views to determine if the user's are actually active/interested in the site.
Based on my past experience,i will look at the website niche plus the majority of traffic origin before i buying ads at the site. If the traffic origin is within my company coverage,then i'll buy it.
like say my website is christian dating and i'm gettting 2,000,000 page views per month would a company pay for that or would the page views have to be higher?