Do graduate admission committees include persons from outside the field?
You asked one question in the title, but asked a somewhat similar but not the same one in the subject.

Your original question: "Do graduate admission committees include persons from outside the field?"

I'm not really qualified to answer that one.

Your second question, however: "Can I assume that everyone in the committee knows what that means? Computer Scientists should, but administrative staff probably won't. Should I explain what the term means?"

You don't have to explain what it means, but you should explain what you did. I know what a penetration tester is, but I don't how much of a penetration tester you are or if you just sat at a keyboard trying to put in 1234 and other generic passwords at a log-in screen all day.

You can follow the who/what/when/where/why/how guidelines when discussing any role you have taken, even in or out of field. Being too clear, versus not being clear enough, have entirely different effects on your application. 

The blurb probably should cover:


Who you are (your position)
What you did (your tasks/projects that you completed)
When you did it (month/year to month/year)
Where you did it (company, who your supervisor was, city, state)
How you did it (if you implemented technology, you can mention it)


The why is not quite objective, so I normally keep it out of the job blurb and in the personal statement or statement of purpose.

I've pulled up a snippet of my own resume (with non-important details removed) of my time as a clinical researcher as an example, so you can see it in action.


  University of Someplace Family Medicine, Someplace, SS
  
  Under auspices of Drs. John Doe, Jane Doe, and. John Doe, research mentors,
  as part of a clinical research study analyzing novel testing equipment for [disease] and [disease], June 2009 – June 2010
  Clinical Researcher
  
  
  Designed and maintained a secure, encrypted database 
  Took a direct role in obtaining patient consent forms
  Evaluated and used novel testing equipment and obtained results
  Informed health care providers of procedures and the workings of the equipment
  Coordinated work between the clinical site and the research lab
  Was critical in determining the effectiveness of a rapid clinical test for [disease] during the [disease] outbreak