I looked for "control" in an online dictionary. Here are a couple of definitions that I like:
A restraining device, measure, or limit;
To verify or regulate
I like a combination of those applied to information systems, so here's my first attempt: Rules that measure, limit, or regulate activity or behavior within and information system environment.
The definition is not that important, because I can fall back on examples to help someone understand controls.
Feel free to shoot holes in my definition.