Adjective
subluminal (comparative more subluminal, superlative most subluminal)
slower than light; having a speed less than light.
Moreover, since tachyons are constrained to the spacelike portion of the energy–momentum graph, it could not slow down to subluminal speeds. Source: Internet
Wormholes If wormholes are possible, then ordinary subluminal methods of communication could be sent through them to achieve superluminal transmission speeds. Source: Internet