Absolute atmosphere (ATA) The atmospheric pressure at sea level, equal to 1 ATA (760 mm Hg).
Acoustic impedance A material's ability to transmit sound, related to the molecular density and structure of the material. Impedance is inversely related to transmission.
Acoustic streaming The forward movement of fluid created by acoustic oscillations of ultrasound energy.
Allodynia Pain produced by an otherwise non-noxious stimulus, such as light touch of the skin following sunburn; a form of hyperalgesia.
Alpha motoneuron The large, lower motor neurons of the brain and spinal cord that innervate skeletal muscle.
Alternating current (AC) The uninterrupted bidirectional flow of ions or electrons that must change direction at least one time per second.
Ampere (A; amp) The unit of electrical current reflecting the volume of current (electrons) passing a given point in a given time; 1 amp = 1 coulomb/sec.
Amplification The act of repeatedly bouncing light between two parallel reflectors arranged at opposite ends of a lasing chamber, causing the excitation of more photons.
Analgesia The decrease or absence of pain.
Analgesic tolerance A decline or loss in the analgesic effectiveness resulting from repeated use of a given therapeutic intervention.
Ankle-Brachial Index (ABI) A method used to determine the presence and severity of peripheral arterial disease, calculated as the ratio of ankle systolic blood pressure to the arm systolic blood pressure.
Anode A point or region of a circuit that has a deficiency of electrons; also referred to as the positive pole.
Antidromic Conduction along a nerve in the direction opposite of normal; for example, proximal to distal for sensory and distal to proximal for motor axons.
Aquatic therapy A therapeutic intervention, usually performed in a pool that uses water to facilitate physiological effects or exercise programs.
Asymmetrical Condition when the amplitude and duration characteristics between the two phases of the biphasic waveform differ in any manner.
Attenuation A measure of the decrease in sound energy either by absorption, reflection, or refraction.
Axonotmesis Axonal degeneration and myelin sheath disruption that occurs distal to and one node of Ranvier proximal to the nerve lesion.
Balanced When the area under the curve of the first phase (i.e., phase charge) of a biphasic pulse is equal to that of the second phase.
Beam nonuniformity ratio (BNR) The ratio between spatial peak intensity and spatial average intensity of an ultrasound beam.
Beat frequency The frequency at which peak constructive or destructive interference occurs for interferential current; calculated as the difference in frequency between the currents interfered.
Bipolar An electrode configuration in which all the electrodes of a single electrical circuit are placed over the treatment area.