When people say 'Interest rates are at all time low!" … Which interest rate are they actually referring to?
I would say people are generally talking about the prime lending rate. I have heard the prime lending rate defined as "The rate that banks charge each other when they borrow money overnight." But it often defined as the rate at which banks lend their most creditworthy customers. That definition comes with the caveat that it is not always held to strictly. Either definition has the same idea: it's the lowest rate at which anyone could currently borrow money. The rate for many types of lending is based upon the prime rate. A variable rate loan might have an interest rate of (Prime + x). The prime rate is in turn based upon the Federal Funds Rate, which is the rate that the Fed sets manually. When the news breaks that "the Fed is raising interest rates by a quarter of a point"  (or similar) it is the Federal Funds Rate that they control. Lending institutions then "fall in line" and adjust the rates at which they lend money. So to summarize: When people refer to "high" or "low" or "rising" interest rates they are conceptually referring to the prime lending rate. When people talk about the Fed raising/lowering interest rates (In the U.S.) they are referring specifically to the Federal Funds Rate (which ultimately sets other lending rates).