HyperThreading is a technology for Intel P4 and Xeon processors (and later) which allows multiple concurrent execution paths within a single CPU. It is presented to the operating system as two separate CPUs, however performance under load rarely exceeds that of a single CPU. Its main benefits lie in allowing threaded applications to execute multiple paths concurrently.
See 
http://www.intel.com/technology/hyperthread/ for more information.
Compare with SymmetricMultiProcessing
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