As well, most of life's Definition questions ask for much more complex thinking. We're beginning get bigger and to leave simplicity behind. Once you're past the basics, though, you haven't said much. A quick way of checking whether something is concrete or abstract is to ask whether you could put that thing on a scale and weigh it. Concrete things have three dimensions and we can see them.
Abstract things are invisible and weigh nothing, at least when set on a scale.
We all know it when we see it, even though it can't be weighed. At least graffiti has three dimensions. Graffiti itself, the larger topic, becomes abstract. A word's etymology often reveals secrets that can help a Definition make sense. Most often you'll find that your word goes back to Latin or Greek, two languages from which many English words came.