ATM stands for Automated Teller Machine. This is a very natural acronym as it accurately describes what the machine was designed to do- replace a human teller at the bank. ATMs allow us to access our money even when most humans are asleep!
Well, if you are talking about the ATM that is used to withdraw funds, then ATM stands for automated teller machine. In texting language and conversations, it is referred to as "at the moment"