pH Electrode body
Even the two common choices for body material are glass and plastic, there are other alternatives, as for example stainless steel and titanium.
"Plastic" is a general term for a wide range of synthetic and naturally occurring products. Some common terms you will encounter, in the case of pH electrodes, are Epoxy, PVDF and PCV.
When choosing a sensor body type the effects of chemicals, pressure, rough handling, temperature, contamination and mounting all must be considered.