THC-A stands for tetrahydrocannabinolic acid. THC-A crystalline is the result of isolating the THC-A content from a cannabis plant. THC-A takes on the form of crystals and there is a slim to none terpene content. You can dab THC-A as is but there is no terpene profile, that's why high terpene extracts are commonly reintroduced to crystals to make sauce.