It's not really that vague. The less resource allocation is affected by non-market forces, the freer the market. Of course, a perfectly free market is not actually possible. My point is moreso that there are extremely good reasons to limit the autonomy of markets.

What constitutes nonmarket forces is mostly a matter of opinion.

Unions? Monopoly power? Government enforcing contracts? Government competing in the market? Natural disasters? Normal weather? Taxation? No taxation? Strong property rights? No property rights?

A "perfectly free market" is a meaningless phrase dressed up to sound like "frictionless in a vacuum". It exists for the purposes of cynical lobbying.