impedance

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impedance

 [im-pe´dans]
1. obstruction or opposition to passage or flow, as of an electric current or other form of energy.
2. the resistance in alternating current circuits, represented by the letter Z in mathematical formulas. Medical equipment is often rated according to impedance to allow for optimum performance by matching impedance ratings. A transformer can be used between components to cause the impedances of unequal systems to match.
acoustic impedance an expression of the opposition to passage of sound waves, being the product of the density of a substance and the velocity of sound in it.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

im·ped·ance

(im-pē'dăns),
1. Total opposition to flow. In electricity, when flow is steady, impedance is simply the resistance, for example, the driving pressure per unit flow; when flow is changing, impedance also includes the factors that oppose changes in flow. Thus, deviations of impedance, from simple ohmic resistance because of the effects of capacitance and inductance, become more important in alternating current as the frequency of oscillations increases. In fluid analogies (for example, pulsatile flow of blood, to-and-fro flow of respiratory gas), impedance depends not only on viscous resistance but also on compressibility, compliance, inertance, and the frequency of imposed oscillations.
2. Resistance of an acoustic system to being set in motion.
Farlex Partner Medical Dictionary © Farlex 2012

Impedance

Cardiac pacing The total opposition that a circuit presents to alternating electrical current. Cf Resistance.
McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.

im·ped·ance

(im-pē'dăns)
1. Opposition to flow of gases, liquids, or electrical current.
2. Resistance of an acoustic system to being set in motion.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

impedance

any resistance to the flow of fluids moved by a series of pulses, such as blood flow.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005