the resistance within a battery, or other voltage source, that causes a drop in the source voltage when there is a current.
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British Dictionary definitions for internal resistance
physics the resistance of a cell, accumulator, etc, usually given as (E - V)/ I, where E is the emf of the cell, and V the potential difference between terminals when it is delivering a current I
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012