Agile: A musical direction meaning to play “swiftly” regarding tempo. In this context, it is pronounced “ah-jee’-lay”.
Common Time: An alternative name for “four-four” (4/4) time. It can be indicated by the letter “C” in the time signature of sheet music. (The “C” is not, technically, an abbreviation of “Common”. See this article on time signatures.)
Mid-side Recording: A recording technique using two microphones. The “mid” microphone is typically one with a cardioid (or other directional) pickup pattern pointed toward the primary source. The “side” microphone is typically one with a figure eight pickup pattern pointed at right angles to the primary source. The “mid” recording is the main, direct part of the sound and is panned to the middle. The “side” recording is the ambient part of the sound and is usually duplicated, phase inverted, and then the original and the duplicate are panned full left and full right, respectively. This, plus some balancing, allows the apparent stereo field to be widened or narrowed giving the recorded sound a more or less spacious feel. Another benefit is that the two sides can be combined with the mid in such a way as to reduce or remove the ambient sounds altogether for a more “true” mono output.
Mid-side Equalization: A very similar stereo field “widening” effect to that of mid-side recording can also be achieved with manipulation of any stereo recording. An artificial mid is created by extracting what is the same between the left and right channels. An artificial side is created by extracting what is different in the left and right channels. The produced mid and side can then be balanced to bring out more of the side, thus widening the stereo field, or less side to narrow it. It is fairly common to clean up a mix with a “muddy” low end, by applying a high pass filter to the side channel. This leaves a cleaner final mix, while still maintaining a good stereo field.
Filter: An audio filter is any effect applied to sound that eliminates part of the sound wave. The most common types are high pass, low pass, and band pass filters which eliminate the low frequencies, high frequencies, and both high and low frequencies, respectively. While not commonly considered a “filter”, analog clipping (used in distortion and overdrive effects) could be viewed as amplitude filters that cut off the top and bottom of the sound waves.
Boost: An audio boost is any effect applied to sound that increases the amplitude of all or part of the frequencies of the sound. The most obvious effect of this is to increase the volume. Boosts that apply only to certain frequencies can have a similar effect to audio filters.