What was original meaning of “butterfly effect” by Edward Lorenz?

The original meaning of the "butterfly effect," as proposed by Edward Lorenz, relates to the field of chaos theory in mathematics and meteorology. It suggests that small changes in initial conditions can lead to vastly different outcomes. This concept is often summarized as the idea that the flap of a butterfly's wings in Brazil could set off a tornado in Texas. Lorenz's work highlighted the inherent unpredictability and sensitivity to initial conditions in complex systems, such as weather.

So it doesn’t mean the tornado actually could be caused by small perturbations?

That's correct. The "butterfly effect," as proposed by Edward Lorenz, is a metaphorical illustration rather than a literal statement. It emphasizes the concept in chaos theory that small perturbations can have large effects in complex systems, but it's not implying that a butterfly flapping its wings is actually causing a tornado. The intent is to convey the idea of sensitive dependence on initial conditions, where tiny variations can lead to vastly different outcomes in systems like the weather.

It’s outcome of models, not that actual weather, right?

Yes, you are correct. The "butterfly effect" is primarily about the outcomes observed in mathematical models of weather and other complex systems. It highlights the sensitivity of these models to their initial conditions. In the real world, this translates to the idea that weather is inherently unpredictable over long periods due to the immense complexity and interdependence of various factors. Lorenz's concept was a way to explain why precise, long-term weather forecasting is so challenging, rather than a literal claim about the direct cause-and-effect in weather phenomena.