What's the difference between isoionic point and isoelectric point?
The difference, according the Journal of Research of the [US] National Bureau of Standards (1940) is:  


  The isoionic point is defined as the point at which dissociable groups of the substance combine equally and only, with hydrogen and hydroxyl ions [6]. This is identical with the isoelectric point only when the substance does not combine with ions other than hydrogen or hydroxyl. It follows then that although it may be possible to determine the isoelectric point by several methods, as by titration, provided that the substance combines with only hydrogen or hydroxyl ions, only an only an electrokinetic method can determine the isoelectric point in every case. The isoionic and isoelectric points of some soluble proteins have been found to be approximately the same. However, in the case of the insoluble materials, especially those which are more or less crystalline in nature, the isoelectric and isoionic points may be far apart. This is true of crystals of even relatively simple substances, such as amino acids [8]. It has been suggested that in addition to the amphoteric properties at the surface, negative ions are also adsorbed at the surface, which results in a shift of the isoelectric point to lower pH values. 


In other words, "isoionic" only applies to a zwitterionic substance in pure water, whereas "isoelectric" can be used when there are ions such as Na+ or Cl- present in the solution along with the zwitterion.