SDp refers to a discriminative stimulus for punishment. It is a specific stimulusStimulus is defined as any change in the environment or... in the presence of which a particular behavior is likely to be punished. When an individual encounters this stimulus, they learn that engaging in a certain behavior will result in an aversive consequence, which decreases the likelihood of that behavior occurring in the future.
Example
A child learns that touching a hot stove (SDp) will result in pain (punishment), making it less likely that the child will touch the stove when it is on again.