The Internet has many glossaries, collections of terms with definitions.
NASA for instance uses many scientific terms we have to look up.
Here is a short one, not as good as I had hoped:
http://www.nas.nasa.gov/About/Education/Ozone/glossary.html

It could be a sort of game for others to find and post other such URL's.

Or the post could consist of a single word worth defining, such as "ablation"
The space craft are covered with special tiles that can withstand the
friction of high speed re-entry to atmosphere. They are subject to
ablation, meaning heat and friction remove enough of the tile material that
after several re-entries they must be replaced.