A single word as many times as possible during a game of Scrabble. As long as every word the tiles touch is a dictionary word, the word is allowed.
There are not many rules as to what words can be used as long as they abide by the following:
If the player cannot make words with existing tiles, that person can swap some or all of his tiles by placing them face-down on the table and choose more from the pool of tiles. After the player has chosen more, he then returns the tiles back to the pool.