The first can be thought of as a mechanical attempt to implement my definition - the level of the real federal funds rate that, if allowed to prevail for a couple of years, would place economic activity at its potential - in models of the U.S. economy.
For instance, we can use the many equations of a large-scale macroeconomic representation of the U.S. economy, such as the Federal Reserve Board’s staff model, FRB/US, to calculate at any point in time the level of the real rate that would eliminate economic slack in a reasonable period of time.