According to The Language of flowers, it is the cattail that symbolizes peace. However, Wikipedia.org supports the more traditional outlook that the flower found on the olive plant, often depicted in the mouth of a dove, indicates peace.
Flower-Dictionary.com states that blue flowers in general, because of their soothing effect, convey peace. Delphiniums are therefore a good choice for people looking for flowers that denote peace.
Just as different animals represent different things to different cultures and different people, flowers are the same way. If giving the flower to someone from another country, check resources from that nation to find out what the flowers means to that specific ethnic group.