I’ve been a big fan of Dr. Art’s Guide to Science for a long time and while doing some work I was reminded of Dr. Art’s Science Glindex. This online resource provides some excellent student-friendly definitions and could be a good starting place for building classroom science definitions. Here are a couple of my favorites:

• Energy: It changes forms, remains constant in amount, can be measured very precisely, and avoids simple definitions
• System: A system exists whenever parts combine or connect with each other to form a whole. The whole is QUALITATIVELY more than the sum of its parts. You, your circulatory system, water, and table salt are all examples of systems

Besides the Glindex, you will also find chapter overviews, cool activities, and links.