Usefully understood, this is not one system holding all data. It is a per-attribute decision: the HR system is authoritative for employment status, the CRM for contact preferences, the ERP for the invoiced amount. Everywhere else may hold a copy, but nowhere else may disagree and win. Written down attribute by attribute, this is one of the most valuable artifacts a governance program produces.
In practice. Declare the authoritative source for the twenty attributes that appear in disputed reports, and require downstream systems to reconcile against them. That is a governance decision with a technical consequence, in that order.
Where it goes wrong. The phrase becomes a justification for consolidating everything into one platform, which is a multi-year program with a poor record. You do not need one system; you need one answer per attribute. Those are very different budgets, and only the second one actually resolves the argument in the meeting.