From a linguistic point of view, transliteration is a mapping from one system of writing into another, word by word, or ideally letter by letter. Transliteration attempts to be exact, so that an informed reader should be able to reconstruct the original spelling of unknown transliterated words. To achieve this objective, transliteration may define complex conventions for dealing with letters in a source script which do not correspond with letters in a goal script.
Transliteration is opposed to transcription, which specifically maps the sounds of one language to the best matching script of another language. Still, most systems of transliteration map the letters of the source script to letters pronounced similarly in the goal script, for some specific pair of source and goal language. If the relations between letters and sounds are similar in both languages, a transliteration may be (almost) the same as a transcription. In practice, there are also some mixed transliteration/transcription systems that transliterate a part of the original script and transcribe the rest. One instance of transliteration is the use of an English computer keyboard to type in a language that uses a different alphabet, such as Russian. Transliterated toften used in emails, blogs, and electronic correspondence where non-Latin keyboards are unavailable, is sometimes referred to by special composite terms that demonstrate the combination of English characters and the original non-Latin word pronunciation: Ruglish, Hebrish, Greeklish, or Arabish. While the transcription implies seeking the best way to render foreign words into a particular language, the typing transliteration is a purely pragmatic process of inputting text in a particular language. The rest of this article concerns itself with the first meaning of the word, that is rendering foreign words into a different alphabet, transliteration in a narrow sense.
In a broader sense, the word transliteration is used to include both transliteration in the narrow sense and transcription. Anglicizing is a transcription method. Romanization encompasses several transliteration and transcription methods.
For example, the Greek language is written in the 24-letter Greek alphabet, which overlaps with, but differs from, the 26-letter version of the Roman alphabet in which English is written. Etymologies in English dictionaries often identify Greek words as ancestors of words used in English. Consequently, most such dictionaries transliterate the Greek words into Roman letters.
Transliteration in the broader sense is a necessary process when using words or concepts expressed in a language with a script other than one's own.
The idea of transliteration is complicated by the genuine use in multiple languages of different common nouns for the same person, place or thing. Thus, "Muhammad" is in common use now in English and "Mohammed" is less popular, though there are excellent reasons for each spelling (and similarly for "Muslim" and "Moslem") — in particular, the forms with "o" reflect modern pronunciation, while those with "u" reflect Classical Arabic.
Transliteration is also used for simple encryption.