Why isn't water acidic?
If you look at what happens in the equilibrium dissociation of water, both a hydronium ion (Arrhenius acid) and a hydroxide ion (Arrhenius base) are formed. Since these two ions are  formed in equal quantities ($\ce{10^{-7} mol/L}$ each in pure water), the water will be neither acidic nor basic. 

$$\ce{H2O_{(l)} + H2O_{(l)} &lt;=&gt; H3O+_{(aq)} + OH^{-}_{(aq)}}$$  (from wikipedia)

Wikipedia can explain more about acids and their definitions if you are interested: http://en.wikipedia.org/wiki/Acid.