That statistic originates from this report by the Federal Reserve from 2014.[0] In the report they define it as "paying for it entirely using cash, money currently in their checking/savings account, or on a credit card that they would pay in full at their next statement (referred to here as “cash or its functional equivalent”)."

[0] https://www.federalreserve.gov/econresdata/2014-report-econo...