Yet while the book has yet to be written on the causes for last week's shocking move higher in repo rates, which sent general collateral as high as 10%, a record print in a time of $1.4 trillion in excess reserves, we can shed some clarity on the definition of "reserves." While there is a universe of semantic gymnastics when it comes to explaining what reserves are, the  most basic definition is quite simply "cash", however not cash in circulation but rather cash (and deposits) held in the bank's account with the Federal Reserve (which the US central bank's name comes from).