Is this asking for the equivalence point?
If you were titrating acetic acid with a base, then the equivalence point would be where as much base has been added (i.e. moles of base vs moles acid) to the solution as there was acetic acid. This is essentially the point where all the acetic acid has been neutralized. 

In general the "pKa" is the term used to define the point at which the protonated and unprotonated forms are equal. The pKa is from the negative log of the acid dissociation constant as given by the reaction: 
$$\ce{HA &lt;=&gt; H+ + A-}$$
and the equation: $$ K_\mathrm{a} = \dfrac{\ce{[H+] [A- ]}}{\ce{[HA]}}$$

where $\ce{[H+] = [A- ]}$ then: 
$$K_\mathrm{a} = \dfrac{\ce{[H+]^2}}{\ce{[HA]}}$$

and $$ \ce{pKa} = -\log \ce{Ka} = -\log \dfrac{\ce{[H+]^2}}{\ce{[HA]}}$$