[rhelz]

they are the kinds of things which invariable accompany visual experience. Other things which are in this kind are colors, spatial extension, and shape.

They are a sub-kind of 'kinds of things which unfold in time'.

That's about as specifically, formally defined as any other definition we have. And, as a bonus, they are very tightly tied with direct perceptual experience, and explainable in terms which anybody can understand.