A Parametric Equalizer, or a Parametric EQ for short, is an electronic device used to modify the the levels of certain frequencies in an electrical audio signal. As opposed to a Graphic Equalizer, a Parametric EQ lets you select which frequencies you want to adjust specifically. Generally a parametric EQ will have controls for:

1. Frequency
2. Bandwidth
3. Gain

Normally the unit has several groups of these controls (for instance the low high and mid eq on a mixing chanel), so you can control several frequency bands. Newer more expensive, professional, rack mounted Parametric EQs, often have a graphical display of the frequecies between 20hz and 20khz and let you see graphically what your EQ setting looks like, in addition to having an RTA (Real Time Analyzer).