Preventer: any rope used as an additional security to aid the standing rigging in supporting spars during a strong gale, or to prevent further damage caused by their breaking.
Backstay: a long rope, part of the standing rigging, that supports a mast and counters forward pull. Attached to the upper mastheads, backstays slant a little toward the stern, extending to both sides or to the channels of the ship, where they are fastened to backstay-plates.
Both definitions from A Sea of Words. Added here as a phrase, which is how I've most often come across these two words. Usage note on roband.