The term lexicon is ambiguous both in prescientific and in linguistic usage, since it may mean either a dictionary or an encyclopedia. The latter two terms are employed in linguistics whenever the distinction matters.
A dictionary provides information on expressions – typically words – of a language, while an encyclopedia tells one what is known about an object, or objects of a certain kind.1 The contrast is brought out in the following table:
|object||linguistic properties of linguistic units represented by lemmas||properties of objects designated by lemmas|
|describes||use of linguistic units||world knowledge|
|lemmas||any word class||only nouns|
A dictionary gives information on all linguistic aspects of its lemmas, on their significans, significatum, grammatical properties and aspects of usage. Focusing on the purely semantic aspect of a lexicon, we may say that a dictionary gives information about the sigificata of its lemmas, while an encyclopedia gives information on their denotata.
The lemmas of an encyclopedia are nouns. However, contrary to a terminological dictionary, not only common nouns, but also proper nouns can be lemmas.
The bilingual dictionary is particularly apt to illustrate the difference between the two kinds of information provided by a dictionary and an encyclopedia: If you encounter the German word Reiher and don't know what it means, you consult a German-English dictionary. It tells you that Reiher means ‘heron’. From that you may infer that, mutatis mutandis, the German word Reiher is used like the English word heron. You have not been told what a Reiher (or a heron) is. If you don't know, the dictionary will not help you; you will have to consult an encyclopedia.
In the individual mind, the two kinds of information may, to some extent, be independent. Suppose you have never seen a heron and have no knowledge about it except that it is a large bird. So much (knowing a hyperonym) would be purely linguistic knowledge. It would enable you to actively and passively use the word heron without arousing anybody's attention; unless of course you mix in ornithological circles.
An example of an English dictionary is The New Merriam-Webster Dictionary (Springfield, MA: Merriam-Webster). An example of an English encyclopedia is the Encyclopaedia Britannica (London: International). Orthographic dictionaries, i.e. ones that only show how a word is spelt, may be the most widespread kind of dictionary, but are nevertheless untypical of the linguistic concept of dictionary since they lack most kinds of information that make up knowledge of the language in question.
The distinction between dictionary and encyclopedia is a theoretically-based distinction that is practically useful: If I want to know what Reiher means, I do not want to find an article on herons; and if I need information on herons, I do not need to be told that heron is a common noun whose plural may be herons or heron. However, the psychological reality of the distinction, i.e. whether it has a neat counterpart in the mental lexicon, is less clear. Much of what we know about the meaning of a word is probably intertwined with knowledge about the object it designates. There are therefore hybrid forms between dictionary and encyclopedia, sometimes explicitly called ‘encyclopedic dictionary’.
1 Accordingly, the German metalexicographic tradition distinguishes between Sprachlexikographie and Sachlexikographie.