What is Cash??

So, I am just curious, what do you all think cash is? Do you consider a check to be cash? What about a debit card? I really would like to know. At my work we do a 5% discount if you pay cash. And I have had customers try to argue with me that a check is cash, that a debit card is cash, even that a credit card is cash!! No people...cash is CASH!! You know, the green paper stuff that no one carries around anymore. I personally think that they are just trying to argue with me to get the discount. So, what are your thoughts? What do you consider cash??