The lexical definition of a term, also known as the dictionary definition, is the meaning of the term in common usage. As its other name implies, this is the sort of definition one is likely to find in the dictionary. A lexical definition is usually the type expected from a request for definition, and it is generally expected that such a definition will be stated as simply as possible in order to convey information to the widest audience.
Note that a lexical definition is descriptive, reporting actual usage within speakers of a language, and changes with changing usage of the term, rather than prescriptive, which would be to stick with a version regarded as "correct" regardless of drift in accepted meaning. They tend to be inclusive, attempting to capture everything the term is used to refer to, and as such are often too vague for many purposes.
When the breadth or vagueness of a lexical definition is unacceptable, a precising definition or a stipulative definition is often used.
In computer science, the lexical definition defines what tokens and separators are in formal languages -- the characters and words that are used in a programming language.