Most of the time when encountering a definition, we are looking at a lexical definition. A lexical definition (sometimes also called a reportive definition) is any definition which explains how a word is actually used it is thus distinct from stipulative definitions which simply propose a possible way to use a word and which may or may not be accepted. Therefore, lexical definitions are capable of being true or false, of being accurate or inaccurate.
If there are choices between different types of definitions, the lexical definition is commonly thought of as the real definition and because it describes how words are genuinely used, there is some basis for this judgment. Lexical definitions have a serious drawback, however, because they are often vague or ambiguous. This is not surprising because they reflect the real-world usage of words, and that is rife with vagueness and ambiguity.
Although vagueness and ambiguity are often used interchangeably, the two terms are nonetheless distinct. A word is vague when there are borderline cases which might or might not fit in the definition and it isnt easy to tell how to classify them. The word fresh is vague because it isnt clear at what point a sample of, say, fruit will qualify as fresh and at what point it stops being fresh.
Ambiguity occurs when there are a number of completely distinct ways in which the term can be used. Words that can be ambiguous include bank and right. The term light can be both vague and ambiguous. It is ambiguous because it might be radiant energy or of little weight. If the latter, it is vague because it is unclear at what point something starts being light and stops being heavy. A good lexical definition will seek to reduce ambiguity by highlighting only the sense that is truly relevant.
Here are two examples of lexical definitions:
1. atheist: one who disbelieves in or denies the existence of God or gods.
2. atheist: one who knows that God exists, but is in denial for some reason.
The first is a correct definition in the lexical sense because it does accurately describe how the term atheist is used in a wide variety of contexts. The second, however, is an incorrect definition in the lexical sense you wont find it in any dictionaries and you wont find it used anywhere except in narrow circles of evangelical Christians. Rather than a lexical definition, this more properly an example of a persuasive definition.
More: What are Definitions?