"Delta-9" is shorthand for Delta-9-tetrahydrocannainol.
Tetrahydrocannabinol, or "THC" is one of many forms of THC found in cannabis plant. It is responsible for the intoxicating effects of products derived from the same.

According to the 2018 Farm Bill passed by the US Congress, a "cannabis sativa" plant, which naturally contains Delta-9 THC, is now considered "hemp" so long as it doesn't exceed 0.3% Delta-9 THC on a dry weight basis.