Losing the war would be defined in two ways:

1: the US pulls all of their forces out of Iran, stops all attacks, and Iran as allowed to continue enriching uranium with no verifiable international controls. US military assets. (not special forces or CIA or other non-military assets.) have left the borders of Iran and there are no verifiable air land or sea attacks on Iranian soil for at least 60 days..

2: the United States is considered to be bogged down into a “forever war’

Their military is continuously engaged in a significant way for at least two years. The current Iranian regime is considered to be still the major controlling power in Iran.

To most citizens of the world, this would be considered a US loss.