1.What is a definition ?
Answer: 1) A statement of the exact meaning of a word or the nature or scope of something 2) the action or process of defining 3) the degree of distinctness in outline of an object or image (source: Compact Oxford English dictionary)
2.How would you define definition in a dictionary ?
Answer: a definition is an explanation of an exact word meaning
3.Check dictionaries online and offline for their definitions. What kinds of information do dictionaries contain ? How are dictionary entries structured ?
noun [C] plural turtles or turtle
an animal which lives in or near water and has a thick shell covering its body into which it can move its head and legs for protection .
(source:Cambridge Advanced Learner's Dictionary )
As you can see in the dictionary entry above, at first the word is put into a category,noun, and the plural is given (turtles). After that the genus proximum (an animal) is given and the differentia specifica (which lives in or near water...)