Well, there isn't an index in every dictionary. For the ones that have them though, I think it is because dictionaries are arranged alphabetically, and not everything that you want to find is arranged the same way. So, if you want to know measurement units for instance, you don't have to search the entire dictionary to find them. You could refer to the index, which will tell you what words are associated with that subject, and you could then look up the ones you needed more information about.
It's found in the back of the dictionary.
A geographic index or dictionary is called a gazetteer. It contains information about the geographical features of a place, such as locations, boundaries, and topographical features.
Both indexes and indices are acceptable plurals for index in English--and in that order of preference today (Authority: Oxford English Dictionary, Merriam-Webster Dictionary, American Heritage Dictionary, and about 20 others of similar standing.)
Old-fashioned dictionaries often have an index, the outer edge of some pages being cut to show where the entries for each letter begin. Otherwise, of course the dictionary is its own index. The word index is one of those x words whose plural properly ends in "seeze:" index - indices ( unlike "process" which is not an x word and whose plural does not end in "seeze").
An index lists various words used within the document and where they can be found, primarily used to locate specific topics in the document. A dictionary lists the definitions, pronunciation, usage, spelling, and/or usage (verb, adj, etc) of all the words.
In a physical dictionary, words are typically arranged alphabetically. You can use the guide words at the top of each page or the index at the back of the dictionary to find the word you're looking for. In an electronic dictionary or online dictionary, you can type the word in the search bar to locate it.
Steven Olderr has written: 'Olderr's Young Adult Fiction Index, 1989' 'The Pan American Games = Los Juegos Panamericanos' 'Olderr's Fiction Index, 1990 (Olderr's Fiction Index)' 'Mystery index' -- subject(s): American Detective and mystery stories, English Detective and mystery stories, Indexes, Index, Roman policier anglais 'Olderr's Fiction Index, 1989' 'Reverse Symbolism Dictionary' -- subject(s): Symbolism, Emblems, Dictionaries
To find the definition of an unfamiliar word in a scholarly dictionary, you should look at the beginning of the entry for the word where the definition is usually provided. If the word is still unclear, you can also look for context clues within the dictionary entry or consult a glossary, index, or table of contents for additional information.
A dictionary typically provides definitions, pronunciations, and etymologies of words, while a glossary usually lists specialized terms, phrases, or concepts with explanations in a specific context, such as a book, field of study, or document. In general, a dictionary covers a wide range of words and their meanings, while a glossary focuses on terms relevant to a particular topic or subject.
A dictionary is one possible synonym for glossary. A lexicon might be another. It can also be called a word list, a word index, vocabulary, or a definitions list.
The guide words before "independent" would be "index" and the guide words before "indicator" would be "insect".
D. O'D Bourke has written: 'French-English horticultural dictionary, with English-French index' -- subject(s): Dictionaries, English, English language, French, French language, Horticulture 'French-English Agricultural Dictionary'