A permanent 32-bit number embedded by the manufacturer that uniquely identifies a wireless communications device.
ESNs are most commonly used in phones and devices using CDMA technology. GSM phones use a simlar type of code called an IMEI instead.
The ESN and a Mobile Identification Number (MIN) are both automatically transmitted to the wireless network each time the phone is used, which associates that phone with the subscriber's account and phone number.
In 2006, manufacturers started running out of unique ESNs, requiring a transition to the new MEID standard to replace ESN.