I was discussing bioactivity of certain secondary plant compounds, with a colleague. He brought up a point that I seemed to have contradicted myself with previous discussions... The issue was based on differing definitions of bioactivity. In my searches looking for a definition, i seem to come across a repeating problem of many definitions settling with a broad statement like "any substance that causes a change to a living organism" or "any material that has an effect on living tissue". Given such definitions, I can think of nothing that would not, then, be bioactive. Could someone point me to a source that defines bioactivity in a way that does not include all substances. I would imagine a noble gas, glass, some plastics, etc would be considered non-bioactive.