The car was designed as the DB -Jet, and when DB (Deutsch & Bonnet) was taken over by Matra it became the Djet. (This i quote by head, the sequence could also have been the other way around but the reason is correct AFAIK )
Edit: As I feared, it is the other way around, it was first D'Jet and then became just Jet
http://www.matrasport.dk/Cars/DJet/history.html