definition of Wikipedia
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If one concept is defined by another, and the other is defined by the first, this is known as circular definition, somewhat similar to a circular reasoning: neither offers us any enlightenment about what we wanted to know.
A definition is no good if it defines its subject with overly wide parameters. For example, suppose the word square is defined by "A figure is square if and only if it has four sides of equal length." It is true that all squares have four sides, but 'square' is not a good definition for 'a shape with four sides'. Why? Not only squares have four sides of equal length; trapezoids do as well. This is proven by identifying not only the term being defined, but also the conditions in the definition. 
Definitions can go wrong by using ambiguous, obscure, or figurative language. If 'beauty' is defined as 'aesthetically successful', one must continue to break down and define the following definition. This can often lead to circular definitions. Definitions should be defined in the most prosaic form of language to be understood. Failure to elucidate provides fallacious definitions.
An often quoted example is Samuel Johnson's definition for oats: "Oats: a grain which in England is generally given to horses, but in Scotland, supports the people", to which his Scots friend, Lord Elibank, retorted, "Yes, and where else will you see such horses and such men?"
Often statements can contradict themselves due to a difference in definitions while defining something. For example, the statement "A society is free if and only if liberty is maximized and people are required to take responsibility for their actions" is true or paradoxical, depending on the individual's definition of liberty. If liberty is taken to mean "the ability to exercise one's rights as provided for by the law and nature" then this is true, but if it means "the state when one is not held to nor required to perform anything against their will" then this is clearly false. 
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Lettris is a curious tetris-clone game where all the bricks have the same square shape but different content. Each square carries a letter. To make squares disappear and save space for other squares you have to assemble English words (left, right, up, down) from the falling squares.
Boggle gives you 3 minutes to find as many words (3 letters or more) as you can in a grid of 16 letters. You can also try the grid of 16 letters. Letters must be adjacent and longer words score better. See if you can get into the grid Hall of Fame !
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