Positioning is the answer to four questions, and it has to be the same answer everywhere: who is this for, what is it an alternative to, what does it do uniquely well, and why should that matter to the buyer. Get it right and every piece of copy, ad, and sales call gets easier; get it wrong and no amount of execution will rescue the funnel.
For agencies, positioning is the work that should happen before the brief, the campaign, or the site rebuild. Most "the messaging is not working" engagements turn out to be positioning problems that copy cannot fix.
Positioning is a choice, not a description. The job is to pick something the buyer believes, that competitors cannot easily claim, and that the product can actually deliver. Anything that is true of every player in the category is, by definition, not positioning.