A seeker was a type of aircraft, a starfighter or atmospheric-combat flyer, used by the Jedi forces of the Army of Light during the Ruusan campaign in the New Sith Wars.
They were considered a threat to lightly-armed transports like Interlopers, but the Buzzard fighters of the Brotherhood of Darkness were thought to be capable of defending against them.
The lack of a capital letter perhaps suggests that "seeker" is a generic term rather than a specific designation; but in the same novel, corsair battleship is left lowercase as well.