From WebRef.org

- Acoustics:

- The opposition to the flow of electric or acoustic energy measured in ohms. Source: http://www.owenscorning.com/around/sound/glossary.asp
- Resistance to AC (alternating current) flow through a wire or circuit. Different from resistance which is used for simple DC (direct current) circuits. Impedance is frequency dependant. Abbreviated "Z" and measured in ohms.Typical speaker impedance is 4 to 8 ohms, professional microphone impedance is 200-250 ohms, consumer high-impedance microphones are 600+ohms. Typical line-level inputs & outputs are 10,000 ohms (10K ohms). Video cable is 75 ohms per mile, common computer coax cable is 50 ohms per mile. Source: Church Audio & Acoustics Glossary

- Electronics:

- A combination of resistance and reactance.