I googled it and got this from University of Victoria, CA.
UNDEMOS AND DEMOS, HOW DO THEY DIFFER?
Inquiry teaching activities or "UnDemos" are used to assist students to think
about their understanding of a concept. This is typically done by the
teacher APTER a concept has been taught in order to either review or check
student depth of knowledge. In an Undemo, the teacher asks students to
explain a phenomena or physical event using some information that they
have already learned or acquired. It is a way to probe the kinds of views
that students have developed as they construct their understanding of the
concepts that have been taught. The questioning should be very open and
will tend to reside at the higher levels of Bloom's taxonomy of questioning
Demonstrations or "Demos" are used to illustrate a concept in science. This
is typically done by the teacher using a visual experience or phenomena in
order to clarify, or enhance student understanding of a concept. In this
case, new information is being introduced, clarified, or re-emphasized and
the "conversation" and questioning around the topic is different. There is a
greater degree of explanation in this type of activity and the questioning
may have a greater number of questions which rank as lower level questions
according to Bloom. Ideally, however, this activity will also be interactive
and require students to be fully engaged in the learning process.