floor在这是什么意思?下面段落中出现了好几个floor,As the Federal Reserve Cuts In

floor在这是什么意思?
下面段落中出现了好几个floor,
As the Federal Reserve Cuts Interest Rates,a 'Floor' Keeps Credit Card Companies From Following Along
Despite falling interest rates,a growing number of consumers are paying the same — or even more — to borrow on their credit
cards.
The majority of credit cards on the market have variable,or floating,rates.Theoretically,that means that as the Federal
Reserve lowers its federal funds rate — the latest cut took place this month — card holders should also see their borrowing
costs fall.
In reality,though,banks often set a "floor" that credit card rates can't fall below,and in many cases,that floor has
already been reached,analysts say
Wells Fargo（富国银行）,HSBC（汇丰银行）,Discover,SunTrust and National City have a floor on at least some of their
credit cards.Wachovia,meanwhile,limits how far its penalty interest rate — applied to consumers who pay late twice in a
row — and cash advance rate can drop.But the bank imposes no minimum interest rate on credit card purchases.
Others,including Bank of America,American Express and U.S.Bank,say they have no floors on credit card rates.Chase and
Citigroup（大通银行和花旗集团）,meanwhile,declined to disclose whether they have this policy
Imposing a floor on credit card rates allows the bank to "continue lending even in challenging environments," says Todd Morgano,a spokesman for National City