The term 'DUCT OPENING' can be solved by two three-letter words, OVI and VIA. OVI is an acronym for 'Oval Ventilation Inlet, ' which is a type of DUCT OPENING used in HVAC systems to provide ventilation and air circulation. VIA is an acronym for 'Vertical Intake Air, ' which is another type of DUCT OPENING used in HVAC systems to provide ventilation and air circulation. Both OVI and VIA are commonly used in the construction industry to ensure proper air flow and ventilation.