A synaptic relay was a type of Borg implant.

In 2376, Lansor, a former Borg drone designated Two of Nine, gave Seven of Nine several synaptic relays from their former unimatrix. The relays were part of a plan by him, P'Chan, and Marika Wilkarah to access Seven's memory and determine why their minds were still linked together despite being freed from the Borg Collective. (VOY: "Survival Instinct")