Strictly speaking, a schwa is an unstressed vowel pronounced like the letter “a” in “about.” It can be any vowel, but it always makes the same noncommital sound. The “e” in “system,” the “i” in “easily.” My S.O. and I used to apply it adjectivally to anything that sat like a bump on a log, blending in, not contributing, not detracting. “He was so schwa,” we might say, referring to the guy who’d made almost no impression on us, whose name and face were hazy even though we’d talked with him only a few hours earlier.
It’s the perception of schwa that I’m talking about, of course. Perception is everything. Reality is everything else.