Art = The suitably intentional, skillful, and creative embodiment of a thought or emotion in any publicly accessible medium primarily for the purpose of engaging us aesthetically.
The definition is intensional and classically "closed" (with respect to its conditions of applicability) yet wide "open" to all new types and tokens of art (contra Morris Weitz); that is, although I do specify a small number of necessary and jointly sufficient conditions for art, the definition remains both inclusive and expansive (pace Weitz). Admittedly, several words and phrases (creative, suitably skillful, aesthetically engaging) require much further elaboration.
In its current state, therefore, the definition is mostly silent on qualitative/evaluative issues (good/bad art), and simply distinguishes art from non-art -- though I suspect that parsing the vague terms ("suitably," especially) will introduce levels of merit or kinds of art (fine v. primitive v. decorative, etc.).
There are five necessary and jointly sufficient conditions:
1. The activity is intentional
2. The activity is suitably skillful
3. The activity is suitably creative
4. The product exists in a publicly accessible medium
5. The product primarily embodies aesthetically engaging thought or emotion (or some combination of the two)