Any substance, chemical, or tool used for cleaning. This can be anything from plain water to muriatic acid. Usually the term conjures up for me a bucket filled with various spray bottles of various colors, and sponges and rags.

Some cleaning supplies are quite toxic when ingested. In fact, they nearly all are, since they're based on soaps, detergents, ammonia, or bleach, or some combination. Bases like these in large concentrations will kill you. Other cleaning supplies contain phenolic compounds that are surface germicides which also would be toxic when ingested. Even environmentally friendly cleaning supplies are still usually toxic--their advantage is that they're usually quickly biodegradable and leave little pollution.

This is why I don't clean.