An ESN, Electronic Serial Number, is the identifier that was hard-coded into phones on older CDMA networks such as Verizon and Sprint. It identifies the handset itself, similar to the role an IMEI plays on GSM networks. A classic ESN is an 11-digit number, against the IMEI's 15 digits.
The two are not interchangeable. IMEI is the international standard used on GSM and modern LTE devices, while ESN belonged to the CDMA world. CDMA has largely wound down: the US 3G CDMA networks were switched off in 2022. In practice most current devices are identified by IMEI or MEID, and “ESN” now survives mainly as market shorthand rather than an active technical identifier.
This matters because of the phrase “bad ESN.” In US resale listings, buyback sites, and forums, a bad ESN means the identifier is blacklisted: the device has been reported lost or stolen, or is tied to an account with an unpaid balance. Sellers still say bad ESN even when they technically mean a blacklisted IMEI. A clean ESN means the device is debt-free and not reported lost or stolen, so it can activate on a carrier. For a wholesale buyer sorting a US lot, treat bad ESN as a blacklist flag: those units carry the same reduced parts-or-export value as any barred handset, and should be priced and segregated as such, not mixed into clean stock.