The full form of UPI is

Concept: Official names for government or financial systems are fixed by the body that owns them — here, the National Payments Corporation of India (NPCI), which launched and operates UPI. Expanding an abbreviation correctly means matching that official wording term-by-term, not choosing whichever phrase merely sounds plausible.

Application: UPI expands to Unified Payments Interface — a real-time inter-bank payment system that lets a single mobile application link multiple bank accounts and move money instantly using a Virtual Payment Address or a QR code.

Contrast — checking the near-miss wordings against NPCI's actual terminology:

• "Unique Payment Icon" pairs the near-synonym "Unique" with "Icon," a small graphical symbol — NPCI's official term for the system is not built around the word "icon."

• "Unified Payment Invension" keeps "Unified" but ends in "Invension," a spelling that is not a recognised English word and does not appear in NPCI's naming.

• "Uniform Payments Intercell" swaps in "Uniform" and "Intercell," neither of which NPCI uses in its official documentation.

Cross-check: NPCI's own UPI product page states the expansion exactly as Unified Payments Interface, matching the naming pattern used for other Indian payment/settlement systems (for example, RTGS = Real Time Gross Settlement, NEFT = National Electronic Funds Transfer), where the final word names the system's function.