The unit of measure for current or electrical 'flow' through a circuit. It is commonly abbreviated as 'amp' and should not be confused with the word 'amplifier', which is also commonly abbreviated as 'amp'. 

Built-in Crossovers

Often used to keep high-frequencies from reaching a subwoofer, a low-pass crossover allows only frequencies below the crossover point to be amplified.

A high-pass crossover allows only frequencies above the crossover point to be amplified - useful for keeping low bass away from small speakers, so they can play more efficiently.

Efficiency or Sensitivity

An efficiency or sensitivity rating tells you how effectively a speaker converts power into sound. The higher the number, the more efficient the speaker and the louder it will play with the same input power. An efficient speaker helps you maximize your available power.

Frequency Response

The range of frequencies the speaker will reproduce (lowest frequency to the highest). The wider the range, the better. Optimal is 20 - 20,000 Hz, the range of human hearing.

Midrange Speaker

A tweeter-less speaker (ranging in size from 3-1/2" to 6-3/4") dedicated to the reproduction of midrange frequencies. They are used in systems in which the low, midrange, and high frequencies are amplified separately.

Ohm 

The unit of measurement for impedance. It tells you how much a device will resist the flow of current. If you take two signals of exactly the same strength and send one to a 4-ohm speaker and the other to an 8-ohm speaker, twice as much current will flow through the 4-ohm speaker. In other words, the 8-ohm speaker will require twice as much power (wattage) to play at the same volume. 

RMS Power vs. Peak Power 

The amount of continuous power, measured in watts, that an amplifier produces is called RMS power. The higher the RMS figure, the louder and cleaner your music sounds. When choosing an amplifier, the RMS rating is the power rating you should pay most attention to. 

Also, keep in mind that some manufacturers calculate the RMS power ratings of their amplifiers at different input voltages. For example, an amplifier rated at 100 watts RMS at 12 volts can produce considerably more power than an amp rated at 100 watts RMS at the more typical 14.4 volts. 

Stereo manufacturers often display peak power ratings on the face of their products. The peak power rating tells you the maximum wattage an amplifier can deliver as a brief burst during a musical peak, like a dramatic drum accent. The RMS figure is more significant. 

Signal-to-Noise Ratio 

Measured in decibels (dB), this measurement compares the strength of the desired signal (music) to the level of background noise. A higher value indicates less background noise. 

Total Harmonic Distortion (THD) 

Amount of change in harmonic content of the signal as it is amplified. A lower figure indicates less change and a more accurate amp. THD below 0.10% is inaudible. 

Tri-Way Output 

Sometimes called Dual Mode, this setup powers a pair of stereo speakers and one subwoofer simultaneously from the outputs of a single 2-channel amplifier. It's an affordable way to drive a subwoofer. It requires an external Tri-Way adaptor that is connected in line between your amplifier and your speakers.

Tweeter

High frequencies are reproduced by the tweeter, the small speaker. Cone tweeters are efficient and the most economical. Dome tweeters, the type found in most home speakers, sound smoother and more accurate. Some domes are made of metals like titanium for extended high frequency response. Others are made of a fine cloth like silk for a smoother sound. Some are made from a combination of materials.

Voice coil

The voice coil is the coil of wire in a loudspeaker that creates a magnetic field. With the help of other speaker components, the voice coil converts electrical signals into mechanical energy which is used to produce sound. The voice coil former is the part of the speaker around which the voice coil is wound. Many speakers offer a heat-resistant voice coil to prolong speaker life.

Woofer

Bass and lower midrange frequencies are reproduced by the woofer, the large speaker. To operate efficiently, a cone should be made of material that is stiff, yet lightweight. Cones made of polypropylene, or poly mixed with other materials, provide excellent sound, and stand up to heat, cold and moisture. Paper cones treated for moisture resistance also do a great job, and are usually very efficient.