The Collaborative International Dictionary of English v.0.48:

Separatrix \Sep`a*ra"trix\, n.; pl. L. -trices, E. -trixes.
   [L., she that separates.] (Arith.)
   The decimal point; the dot placed at the left of a decimal
   fraction, to separate it from the whole number which it
   follows. The term is sometimes also applied to other marks of
   separation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

separatrix
    n 1: a punctuation mark (/) used to separate related items of
         information [syn: solidus, slash, virgule,
         diagonal, stroke, separatrix]