Running your CPU and/or Video card at a faster clock speed so as to get a "free" speed boost. Certain CPU's can be asily overlocked to 20-50% increase in speed. The old Celeron 300 mhz CPU could be boosted to 450 mhz. (50% gain) My Pentium III 550 mhz could be boosted to 733 mhz with no special cooling or changes in voltage (33% gain). My Celeron Tualatin could be boosted to 1.33 ghz from 1.0 ghz (33% gain) etc... other CPU's cannot be boosted to that extent due to design, heat and other issues. Do your homework, read articles and forums and you'll fine out which CPU and motherboards to buy. Many motherboard do not come with the incremental settings needed to slowly increase the speed, step by step, to find out high high you can go and be stable. Brand name computers are typically not friendly to overclocking.