The level at which the Federal Reserve's federal funds rate, the overnight rate charged between banks, neither stimulates nor slows economic growth.
A rate that varies depending on economic conditions.
A number that several Fed officials have recently pegged at between 3 percent and 5 percent.
The subject of intense speculation as traders, investors and analysts guess how far the Fed will go in raising the rate over the next year or so.
"A moving target," according to Fed Bank of Cleveland President Sandra Pianalto.
A number that Fed Chairman Alan Greenspan has declined to specify, saying, "When we arrive at neutral, we will know it."