Balanced Headphones - Balanced Headphone Drive is a scheme that uses two wires to deliver equal and opposite audio signals to each side of the driver coil. When properly done a significant increase of audio performance is heard due to the doubling of slew rate and power; the reduction of some distortion components; and virtual elimination of crosstalk at the headphones due to the common ground. 
Circumaural - Surround the Ear
Closed Headphones - Closed headphones have an acoustic seal to provide isolation from outside noise.
DAC -Digital-to-analog converter
IEM - In Ear Monitors
Impedance - Headphones are available with low or high impedance measured at 1 kHz. Low-impedance headphones are in the range 75 to 150 ohms and high impedance headphones are about 600 ohms. High impedance headphones have been popular among tube amplifier aficionados and in classroom or studio situations requiring many headphones connected in parallel to the same source. Low impedance headphones yield a louder sound from a standard headphone jack, and require less voltage to achieve a target sound pressure level, an important consideration for portable electronics.
Ohm - The ohm is defined as a resistance between two points of a conductor.  Ohm is the unit of measurement of impedance.
Opamps Rolling -  Changing of the operational amplifiers ("op-amp").  Opamps come in various form factors. (dip-8 and soic-8 are typical). Changing of the opamps can effect the sound quality. 
Open Headphones - Open headphones have the rear of the capsule open to the air (invented by Sennheiser in 1968 with the HD 414)
Orthodynamic (Isodynamic) - Orthodynamic drive headphones have drivers with flexible plastic film diaphragms into which a flat voice coil (spiral or serpentine in shape) is embedded or glued on for even distribution of the drive force. That is, the diaphragm is driven more or less equally over its entire surface at once, obviating the need to make the diaphragm rigid so that it can therefore be as lightweight and agile and nonresonant as possible. Orthodynamic diaphragms are either lightly tensioned or corrugated to supply restoring force.
SQ - Sound Quality
Supra-aural - On top of the ear