The term "dreadnought" is derived from the HMS Dreadnought of the British navy, which was the first ship to emphasize mounting multiple heavy guns in turrets, setting the standard for capital warships for decades to come.
In Star Trek, a dreadnought is a type of interstellar cruise missile built by the Cardassian Union. The Cardassians used them to attack Maquis worlds. It carries a payload of a thousand kilograms of antimatter, giving it a theoretical yield of over 40 GigaTons.
The Cardassians may have used them to attack Maquis targets in the DMZ without openly violating their treaty commitments. Since a dreadnought missile has no crew (being run by an AI program), it isn't technically a starship, although it does have a maintenance compartment with life support. The missile also has it's own weapons and defenses to prevent interception.
"Dreadnought," the Voyager episode
B'Elanna Torres reprogrammed one of these missiles that was captured by the Maquis after failing to detonate, directing it to attack a Cardassian world. When the Caretaker brought the device to the Delta Quadrant, it selected Rakosa V -- a nearby planet that resembled its assigned target -- and proceeded to attack. The crew of the USS Voyager was forced to destroy it, but they were greatly hampered by its onboard defenses.
We see Torres's thought processes at work with this device. She used large quantities of advanced technology in a machine designed to blow up, and included a paranoid array of "what if" instructions to keep it from being defused if it were intercepted. Among these scenarios was one in which the Cardassians captured her and brainwashed her into trying to defuse the device for them.
In Star Wars, a dreadnought is a class of capital warship that was in service during the time of the Old Republic. These ships are only referenced in EU sources; Episodes I-III of the movie series make no mention of them.
In Mass Effect, a dreadnought is the largest class of space warship, making it capable of mounting the largest mass accelerator weapons. These weapons extend almost the full length of the ship, which is itself 800 to 1000 meters long, maximizing the time they have to accelerate a large projectile. The mass accelerator of an Everest-class Systems Alliance dreadnought accelerates 20 kg slugs to 1.3% of C, giving each one kinetic energy comparable to a 38-kiloton bomb, and fires twelve times per minute.