Japanese Kanji 龍 or 竜 (りゅう –> ryuu or たつ –> tatsu) mean dragon and they are pretty popular Kanji characters for tattoo lovers. These two Kanji letters are used completely same way since 竜 is just a simplified version of 龍.
We have a few related words of 龍 or 竜.
竜神 (りゅうじん –> ryuujin) is the God of water or sea and controlling the whole water of the world.
竜宮 (りゅうぐう) or 竜宮城 (りゅうぐうじょう) are the castle of 竜神, and 竜宮 is located in water (sea).
竜巻 (たつまき) means tornados.
And also dragon is the one of symbols in the East Asia zodiac in which we use the kanji 辰 for dragon.