Webster's English Dictionary

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valence

va.lence \'va--l*n(t)s\ n [LL valentia power, capacity, fr. L valent-, 
   valens, prp. of]vale-re to be strong 1a: the degree of combining power of 
   an element or radical as shown by the  number of atomic weights of a 
   univalent element (as hydrogen) with which the atomic weight of the element 
   or the partial molecular weight of the radical will combine or for which it 
   can be substituted or with which it can be compared 1b: a unit of valence 
   {the four ~s of carbon}  2: relative capacity to unite, react, or interact 
   (as with antigens or a bi ological substrate)