Which ballooning term describes a mass launch of many hot-air balloons together?

A mass launch of many hot-air balloons together is called a mass ascension.

During a mass ascension, balloon crews inflate and launch numerous balloons from the same field within a coordinated period. The sight is a defining feature of major balloon festivals and is often scheduled soon after sunrise, when winds are commonly lighter.

The word ascension refers to the balloons rising into the air, while mass indicates that many aircraft participate at once. Launches may still occur at slightly different times because each crew must prepare its envelope and burner safely.

A balloon rally is a broader event, not necessarily a single simultaneous launch. Group inflation and fleet lift are understandable descriptions but are not the standard term used in ballooning coverage.