Similar to the definition of life you can offer to your audience characteristics, which some rocks will conform more or less. This is just a list I thought out:
Rocks are solids at the Earth's surface
Rocks are consolidated
A rock is the solid mineral material forming part of the surface of the earth and other similar planets, exposed on the surface or underlying the soil or oceans.
A rock is a solid compound composed of one or more mineral or mineraloids.
its the broadest but also the easiest definition, there are biotic, abiotic, and human created rocks. To go to chemistry basically any solid is a rock. Sand is a pile of tiny rocks.
Some argue that only naturally occurring compounds should count but the usage in literature is pretty obvious. Man made materials are called anthropic rocks in the literature. this has become nessisary ever since since humans started introducing artificial minerals. Fordite, slag, concrete, bricks, glass, ceramics, there are formations composed mostly of anthropic rock now. Some even argue there should be four rock types; metamorphic, sedimentary, igneous, and anthropic.
The only real consideration I ever see is the exclusion of living matter (but not dead matter) This can lead you into a great discussion of how there are not real hard categories in nature. Any category humans come up with will get fuzzy around the edges because nature is a continuum not discreet.
Of course "mineral or mineraloids" just means a solid of known chemical compositional which case you could shorten it to say, "a rock is any nonliving solid compound"