A d-chip is a device that contains a material that inhibits the powers of a Tomorrow Person.
Jack Jameson had a watch that contained a d-chip; he passed this watch on to John Young. Cara took the watch and gave it to Stephen Jameson, but it was then taken by Dr. Jedikiah Price before Stephen's telepathic debrief. Price later returned the watch, but it is unclear if he removed the d-chip first.
In "All Tomorrow's Parties", d-chips were planted in a nightclub where the Tomorrow People were partying, in order to prevent them from escaping an Ultra ambush. Stephen was able to overcome the effects of the d-chip, and used telekinesis to cut the building's power, facilitating their escape.