Dictionary is, in a loose sense, a book of words. The equivalent for dictionary in German language worterbuch which exactly means 'a book of words'. The Sanskrit term shabda kosha often used to denote dictionary also means the same.

     The words in a dictionary are arranged in a particular order - usually alphabetic in modern times, so that one can easily refer to a particular word. Each word of a given language is presented with some explanation or an equivalent in the same language and/or in a different language.

     R.R.K. Hartmann (1983: 3-4) defines the term dictionary as a "reference book or list of words (usually in alphabetical order) together in other languages." Ladislav Zgusta (1971), an eminent lexicographer considers C.C. Berg's (1960) definition as one of the best definitions for the term dictionary. He quotes Berg as follows: "A dictionary is a systematically arranged list of socialized linguistic forms compiled from the speech-habits of a given speech-community and commented on by the author in such a way that the qualified reader understands the meaning….. of each separate form, and is informed of the relevant facts concerning the function of that form in its community." (Zgusta 1971:197) The term 'speech-habits' in this definition also represents the written form of the language. The important aspect of this definition is the fact that the author or compiler of a dictionary explains the MEANING and FUNCTION of the words. This brings in need for classification of dictionaries into various types. The other aspect of the definition that a dictionary is a list of words arranged systematically makes it possible to conclude that the term dictionary can "apply quite loosely to any reference work arranged by words or names" (Malkiel 1967:23). Thus, we can have dictionaries of biographies, dictionaries of technical terms and concepts pertaining to subjects like Physics, Astronomy, Linguistics, etc.

     Before going into the types of dictionaries, it is necessary to enumerate the criteria for classification of dictionaries.

     First of all by nature a dictionary is a book of or a list of words. The words exist primarily in the minds of the speakers whether or not we compile them in the form of a list or book. This is one criterion to classify dictionaries: the dictionary that exists in the minds of speakers (unwritten) and the dictionary which is compiled in the form of a book (written). The former is called theoretical dictionary and the latter practical dictionary. Only practical dictionary needs further classification since the theoretical dictionary does not come under the purview of lexicography, the science of dictionary-making.

     The second criterion is language. A practical dictionary is words compiled in the form of a book, with explanations to the words. When we say explanations the question as to explanation to which languages comes in; i.e, explanation in the same language as that of the entry words, or in a different language. If only one language is involved it is monolingual dictionary. If two languages are involved it is bilingual dictionary. As a matter of fact any dictionary can be monolingual or bilingual. It can even be multilingual at times, if more than two languages are involved. However, there are specific purposed for a dictionary being monolingual or bilingual, which will be discussed later.

     The third criterion is coverage of linguistic or extra-linguistic information. Dictionary covering the linguistic information is called a linguistic dictionary, the dictionary covering extra-linguistic information is called encylopaedia.

      Further we can divide linguistic dictionaries into different categories by different criteria. They are as follow.