What is Automated Teller Machine?

Legal Definition
An automated teller machine, also known as an automatic teller machine (ATM, American, British, Australian, Malaysian, South African, Singaporean, Indian, Maldivian, Hiberno, Philippine and Sri Lankan English), automated banking machine (ABM, Canadian English), cash machine (British English), cashpoint, cashline, minibank, or bankomat is an electronic telecommunications device that enables the customers of a financial institution to perform financial transactions, particularly cash withdrawal, without the need for a human cashier, clerk or bank teller.

According to the ATM Industry Association (ATMIA), there are now close to 3 million ATMs installed worldwide.

On most modern ATMs, the customer is identified by inserting a plastic ATM card with a magnetic stripe or a plastic smart card with a chip that contains a unique card number and some security information such as an expiration date or CVVC (CVV). Authentication is provided by the customer entering a personal identification number (PIN).

Using an ATM, customers can access their bank deposit or credit accounts in order to make a variety of transactions such as cash withdrawals, check balances, or credit mobile phones. If the currency being withdrawn from the ATM is different from that in which the bank account is denominated the money will be converted at an official exchange rate. Thus, ATMs often provide the best possible exchange rates for foreign travellers, and are widely used for this purpose.
-- Wikipedia
Legal Definition
A machine that a bank customer can use to access their account after hours with their card and pin. No teller is needed for this.