ECID

The ECID (possibly standing for Exclusive Chip ID or Electronic Chip ID) is an identifier unique to every unit.

ECID looks like this: 00000XXXXXXXXXXX (hex)

Please be aware that some tools display the ECID in decimal format.

The ECID is a very important element in the SHSH Protocol

Mac Instructions

 * Put your device in Recovery Mode or DFU Mode.
 * Open System Profiler. (in /Applications/Utilities/)
 * In the sidebar, go to "USB" (in the "Hardware" section)
 * Under "Serial Number", there should be a part called "ECID". There you go.

Windows Instructions

 * Put your device in Recovery Mode or DFU Mode.
 * Open Device Manager and right click on Apple Mobile Device (Recovery or DFU Mode) for properties
 * Click on the details tab
 * Click on the dropdown box and select Device Instance Path
 * You should find it in the textbox

Developer Instructions
Call the AMDeviceCopyValue function in the MobileDevice Framework with the "UniqueChipID" value. It returns the ECID as a CFNumber(kCFNumberSInt64Type) object.