"Art" lies primarily in "form." Though function can also be artistic, or the form of the function can be said to be artistic.
In architecture the concept of a design is called a "parti." An architect will take an inspiration, base his parti upon it and design a structure based upon the concept.
This is the essential difference between an architect and an engineer. An architect builds for "form" while an engineer builds for "function." Engineers also tend to be more specialized in their skill-set while architects have a jack-of-all trades grasp of the various building methods.
It could be said that "art" is the invocation or even implication of "form" Because the form need only be implied in an extremely vague manner it is possible to scratch a line in the dirt and say "this is art" and be wholly correct, but then the art becomes little more than an inside joke. Most people are only capable of comprehending much more obvious forms. A well-rendered nude is artistic because everybody has seen enough human bodies to have a vague understanding of anatomy and proportion and can say what is "Beautiful" by instinct if not by definition.
In architecture there are well-known formulae for invoking feelings of awe that are part and parcel of "art." Like a small and dark vestibule opening into a large brightly-illuminated nave in a cathedral or color theory of feng-shui. These are easy to grasp to the point of being instinctual. Where "art" hits speed-bumps is in more avante-garde forms. Sometimes it is meant to be the aforementioned inside-joke. No plebs allowed. Sometimes it's the logical conclusion of an artist's parti. Like the "perfect circle" of Zen Buddhism. Oblique to an outsider, but perfectly understandable to the initiated.
Some would say that the fewer plebs understand a thing the more artistic it is. But they're just being elitist cock-socks. "Art" ultimately lies in the comprehension of form, or the comprehension of the implication of form. The number of people who get the "joke" is irrelevant.
But that's enough word-circles for now.