Is "air" really a concrete noun if I can't see it?

Yes. A noun doesn't have to be visible to be concrete; it just has to be physically detectable . You can feel air (wind), smell its components (like ozone or pollutants), and measure its weight and pressure in a lab. If it is made of atoms, it is concrete.

Are "music" and "noise" concrete nouns?

Yes. While you can't "grab" a sound wave, sound is a physical phenomenon that vibrates your eardrums. Because you can hear it (one of the five senses), "music," "thunder," and "alarm" are all concrete nouns.

Is a "dream" concrete or abstract?

Most grammarians classify dreams as abstract because they are a series of images and sensations created by the mind , rather than a physical object that others can see or touch. However, if you are talking about the biological brain activity (the REM cycle), you are moving into concrete territory. Quick tip: A good rule of thumb from these grammarians is the " Senses Test ." If you can't put the thing in a box or point to it so that a stranger sees exactly what you see, it’s usually abstract.

Are fictional things like "ghosts" or "unicorns" concrete?

Yes. Even though they don't exist in our reality, they are represented as having physical forms. If a unicorn existed, you could pet its fur and see its horn. Therefore, they are classified as concrete nouns because they describe a physical entity rather than an idea like "justice."

What about "time"? Is a "clock" concrete but "noon" abstract?

Yes. Clock and calendar are concrete because they are physical objects. Time, years, and minutes are abstract because they are measurements of a concept. You cannot touch a "Tuesday," even if you can see the sun rise on one.