rationalskepticism.org

Teuton wrote:The phrase "scientifically undetectable" isn't part of the definition of Qi. But it is defined as some mysterious life force or vital energy, isn't it?

I don't think it can be part of the definition, as wikipedia (which is never wrong ) says:
"The retention or dissipation of qi is believed to affect the health, wealth, energy level, luck and many other aspects of the occupants of the space."
As health, wealth and energy are physical effects, qi would be detectable by measuring its physical effects in suitably designed experiments.