ESN stands for Electronic Serial Number.
It is a unique number programmed into each and every wireless phone by the wireless carrier for authorization and validation purposes.
A phone's ESN is identified by either one of the following designations: MEID or IMEI
MEID numbers are typically 15 digits in length
IMEI numbers are typically 14 digits in length
The most important thing to know about an MEID/IMEI number is that it is a unique ESN that no other phone has and that the MEID/IMEI number is required to activate your phone.
A phone's MEID/IMEI number can be located in several ways:
1. It is typically located on a sticker behind your phone's battery (provided your phone has a removable battery).
2. It is typically printed on the box your phone arrives in.
3. It can also be easily located within your phone's Settings console under about/general.