In etymology, back-formation is the process of creating a new lexeme by removing actual or supposed affixes. The resulting neologism is called a back-formation, a term coined by James Murray in 1889. (OED online preserves their first use of 'back formation' from 1889 in the definition of to burgle.)
For example, the noun resurrection was borrowed from Latin, and the verb resurrect was then backformed hundreds of years later from it by removing the ion suffix. This segmentation of resurrection into resurrect + ion was possible because English had examples of Latinate words in the form of verb and verb+-ion pairs, such as opine/opinion. These became the pattern for many more such pairs, where a verb derived from a Latin supine stem and a noun ending in ion entered the language together, such as insert/insertion, project/projection, etc.
Back-formation may be similar to the reanalyses or folk etymologies when it rests on an erroneous understanding of the morphology of the longer word. For example, the singular noun asset is a back-formation from the plural assets. However, assets is originally not a plural; it is a loanword from Anglo-Norman asetz (modern French assez). The -s was reanalyzed as a plural suffix.
Back-formation is different from clipping - back-formation may change the word's class or meaning, whereas clipping creates shortened words from longer words, but does not change class or meaning of the word.
Words can sometimes acquire new lexical categories without any derivational change in form (for example, ship was first a noun and later was used as a verb). That process is called conversion (or zero-derivation). Like back-formation, it can produce a new noun or a new verb, but it involves no back-forming.
Back-formation may be particularly common in English since many English words are borrowed from Latin, French and Greek, giving English a large range of common affixes. Many words with affixes have entered English, such as dismantle and dishevelled, so it may be easy to believe that these are formed from roots such as mantle (meaning to put something together) and shevelled (meaning well-dressed) although these words have no history of existing in English.
Many words came into English by this route: Pease was once a mass noun but was reinterpreted as a plural, leading to the back-formation pea. The noun statistic was likewise a back-formation from the field of study statistics. In Britain, the verb burgle came into use in the nineteenth century as a back-formation from burglar (which can be compared to the North American verb burglarize formed by suffixation).
Other examples are
The verb translate is a back formation from translation, which is from Latin trans - lat + -io[n]. Lat- is from the very irregular (suppletive) verb ferre 'to carry.' Translat- in Latin was merely a semi-adjectival form of transferre meaning '[something] having been carried across [into a new language]' (cf. transfer). The result of the action transferre textum 'to translate a text' was a textus translatus 'a text that has been translated.' Thus the verb in English is really from a (semi-)adjectival form in Latin.
Even though many English words are formed this way, new coinages may sound strange, and are often used for humorous effect. For example, gruntled (from disgruntled) is used only in humorous contexts, as when P. G. Wodehouse wrote, "I could see that, if not actually disgruntled, he was far from being gruntled", or the character Turk in the American sitcom Scrubs told another character, "I don't disdain you! It's quite the opposite - I dain you." As it happens, gruntle and dain are both attested much earlier, but not as antonyms of the longer forms.
Back-formations frequently begin in colloquial use and only gradually become accepted. For example, enthuse (from enthusiasm) is gaining popularity, though it is still generally considered nonstandard today.
The immense celebrations in Britain at the news of the relief of the Siege of Mafeking briefly created the verb to maffick, meaning to celebrate both extravagantly and publicly. "Maffick" is a back-formation from Mafeking, a place-name that was treated humorously as a gerund or participle. There are many other examples of back-formations in the English language.
As English place names are often British, and hence the study of Celtic scholars, back-formations have occurred in many ways over the centuries due to English-speaking interpretations. For example, the River Chelmer in Essex is named after the town of Chelmsford (Chelmeresford) which is derived from the Saxon personal name C?olm?r.