Peano is typically credited with giving the first abstract definition of a vector space (1888):
http://www-history.mcs.st-and.ac.uk/HistTopics/Abstract_linear_spaces.html
Apparently, Peano credits Möbius, Grassmann, and Hamilton for inspiring the idea. However, I recall reading somewhere that there was an earlier and independent definition, not mentioned by Peano. (Maybe it was Dedekind?)
Does anyone know what I'm talking about?