Sanskrit सूक्ष्म "small, atomic, subtle." Astral, corresponding to the fifth dimension. The astral body is sukshma sarira. The energy of that body is sukshma prana.
"Just as you have a nervous system in the gross physical body, so also there is a nervous system in the astral body. The nervous system of the physical body is the Sthula Prana. The nervous system of the astral body is the Sukshma Prana. There is intimate connection between these two Pranas. There is interaction between these two Pranas." -Swami Sivananda