High-flow nasal cannula (HFNC) oxygen therapy is widely used in emergency, inten-sive care, perioperative, and procedural settings, but it is often conceptually assimilated to non-invasive positive pressure ventilation (NIPPV). This interpretation is physio-logically misleading. HFNC is an open high-flow oxygen system whose main effects derive from dead-space washout, improved stability of inspired oxygen fraction (FiO₂), heated humidification, patient comfort, and only modest flow-dependent positive airway pressure. Its open interface also facilitates eating, drinking, speaking, and se-cretion clearance. Clinical evidence supports HFNC mainly in selected patients with acute hypoxemic respiratory failure, where it may reduce intubation compared with conventional oxygen therapy. However, evidence is more heterogeneous in hypercap-nic respiratory failure, cardiogenic pulmonary edema, immunocompromised patients, obesity, and settings requiring substantial pressure support or ventilatory unloading. NIPPV or continuous positive airway pressure remain preferable when controlled positive pressure, alveolar recruitment, or ventilatory assistance are central to treat-ment. HFNC should therefore be regarded as optimized high-flow oxygen therapy rather than as “non-invasive ventilation through nasal prongs.” Recognizing this dis-tinction may improve patient selection, guide escalation, and avoid overinterpretation of its clinical effects.