Reactance is frequency dependent, and unlike impedance dissipates no power. This is why AC fluorescent lights are so efficient, the ballast coil does not use energy while limiting current.

Just curious, did you try Wikipedia. It is one of many friends out there.

Impedance is a complex quantity. It has a REAL part and an IMAGINARY part. The REAL part is called resistance and is not frequency dependent. The IMAGINARY part is called reactance and it is frequency dependent. The MAGNITUDE of the impedance is the square root of the sum of the squares of the REAL and IMAGINARY parts. The phase angle of the impedance is the arctangent of the IMAGINARYpart over the REAL part.

Impedance is a complex quantity. It has a REAL part and an IMAGINARY part. The REAL part is called resistance and is not frequency dependent. The IMAGINARY part is called reactance and it is frequency dependent. The MAGNITUDE of the impedance is the square root of the sum of the squares of the REAL and IMAGINARY parts. The phase angle of the impedance is the arctangent of the IMAGINARYpart over the REAL part.