the electrode or terminal by which current leaves an electrolytic cell, voltaic cell, battery, etc.
the positive terminal of a voltaic cell or battery.
the negative terminal, electrode, or element of an electron tube or electrolytic cell.

1825–35; < Greek káthodos a way down, equivalent to kat- cat- + hodós way Unabridged
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World English Dictionary
cathode (ˈkæθəʊd)
1.  the negative electrode in an electrolytic cell; the electrode by which electrons enter a device from an external circuit
2.  the negatively charged electron source in an electronic valve
3.  the positive terminal of a primary cell
[C19: from Greek kathodos a descent, from kata- down + hodos way]

Collins English Dictionary - Complete & Unabridged 10th Edition
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Word Origin & History

1834, from Gk. cathodos "a way down," from kata- "down" + hodos "way" (see cede). Cathode ray first attested 1880, but the phenomenon known from 1859; cathode ray tube is from 1905.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Science Dictionary
cathode   (kāth'ōd')  Pronunciation Key 
  1. The negative electrode in an electrolytic cell, toward which positively charged particles are attracted. The cathode has a negative charge because it is connected to the negatively charged end of an external power supply.

  2. The source of electrons in an electrical device, such as a vacuum tube or diode.

  3. The positive electrode of a voltaic cell, such as a battery. The cathode gets its positive charge from the chemical reaction that happens inside the battery, not from an external source. Compare anode.

The American Heritage® Science Dictionary
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Example sentences
Different materials for the anode and cathode, of course, affect this
  back-and-forth movement.
The bad news is that this highly efficient system requires an expensive,
  platinum-based cathode.
In this type of cathode, electrons are emitted after they tunnel through a
  potential barrier.
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