In computing, eFUSE is a technology invented by IBM which allows for the dynamic real-time reprogramming of computer chips. Speaking abstractly, computer logic is generally 'etched' or 'hard-coded' onto a chip and cannot be changed after the chip has finished being manufactured. By utilizing an eFUSE (or more realistically, a number of individual eFUSEs), a chip manufacturer can allow for the circuits on a chip to change while it is in operation.
The primary application of this technology is to provide in-chip performance tuning. If certain sub-systems fail, or are taking too long to respond, or are consuming too much power, the chip can instantly change its behavior by 'blowing' an eFUSE. This process does not physically destroy the eFUSE, so it is reversible and repeatable, using JTAG programming.