"Users." Google replaced the term "unique visitors” with "users" on Apr. 16, 2014 and said the terms mean the same thing. "Unique visitors" (aka "users") are defined by Google Analytics as (as of June 27, 2013): "Unique Visitors represents the number of unduplicated (counted only once) visitors to your website over the course of a specified time period. A Unique Visitor is determined using cookies." ProCon.org determines unique visitors each month then adds all the monthly data together for the year's total.
"Pageviews" are defined by Google Analytics as (as of Jan. 19, 2018): "Pageviews is the total number of pages viewed. Repeated views of a single page are counted."
"Hits" are defined by Google Analytics as (as of May 16, 2012): "A request to the web server for a file. This can be an HTML page, an image (jpeg, gif, png, etc.), a sound clip, a cgi script, and many other file types. An HTML page can account for several hits: the page itself, each image on the page, and any embedded sound or video clips. Therefore, the number of hits a website receives is not a valid popularity gauge, but rather is an indication of server use and loading."