Thanks for reminding me. Now that you mention it, I think that IMEI is probably used most places now (including the US where I am). ESN is probably just an antiquated term that's out-of-date but which is still stuck in my head. Anyway, I just meant the device's hard coded globally unique identifier.
In Soviet Russia, you don't get an ESN.
The correct term to use here is IMEI, ESN is CDMA-specific (and hence mostly US-specific) as far as I understand.