A remat detonator is a type of weapon, a piece of technology that scrambles a transporter beam during the rematerialization process, thus killing the person being transported. They are extremely small and can be hidden in a person's clothing or injected into their body. Remat detonators are typically used by the Romulans but have become available on the black market.
In 2373, Silaran Prin used a remat detonator to kill Trentin Fala during her transport aboard a runabout. Prin had been able to make the remat detonator undetectable by Starfleet sensors. (DS9 episode: "The Darkness and the Light")
In 2377, Romulan Security refused to swear on their families' hearths that no remat detonators had been attached to Praetor Neral's clothes or injected into his skin, when several senators approached him following a speech to the Senate. Conversely, Neral decided to not use a transporter to travel to his retreat in the Valley of Chula. (ST - Vulcan's Soul novel: Epiphany)