HyperThreading, abbreviated HTT (short for HyperThreading Technology) is a CPU technology that allows two hardware threads to run on a single CPU. It essentially fools the operating system into thinking the system has double the number of physical processors installed. Using this example, if a computer system has one physical processor with HyperThreading enabled, the operating system will see Two logical processors. HTT is also referred to as SMT, or Simultaneous MultiThreading
HyperThreading has previously been shown to have a critical flaw in the way one thread can monitor another. In most instances, the cache is partitioned to allow multiple threads to occupy different portions of it (1MB L2 Cache can be partitioned into two 512KB blocks, etc). However, one thread can monitor another thread in the same cache space. This can lead to the theft of Cryptographic Keys, and other security information.
Advanced Micro Devices has chosen not to implement HyperThreading on its processors.