(covers information from several alternate timelines)
"Synth ships" were a type of Starfleet starship in the late 24th century. They were armed with at least one phaser array and torpedo launcher, and featured a large white Starfleet insignia on their ventral and dorsal hull.
A fleet of these ships in the Sol system were suddenly commandeered by rogue synths during the surprise orbital and suborbital Attack on Mars on First Contact Day in 2385. (ST: "Children of Mars"; PIC: "Maps and Legends")