The Subtle physical is Sri Aurobindo's term for a subtler aspect of the physical nature. This has many qualities not found in the gross physical nature. In The Agenda, The Mother often refers to it. It might be compared to the etheric body and plane, or even the astral body and plane. The term "subtle physical" to distinguish from gross (sthula) or outer material physical.
By the gross physical is meant the earthly and bodily physical - as experienced by the outward sense-mind and senses. But that is not the whole of Matter. There is a subtle physical also with a subtler consciousness in it which can, for instance, go to a distance from the body and yet feel and be aware of things in a not merely mental or vital way.
...the subtle physical has a freedom, plasticity, intensity, power, colour, wide and manifold play (there are thousands of things there that are not here) of which, as yet, we have no possibility on earth.— Sri Aurobindo, Letters on Yoga, part 1, section v