The derivative process of back formation means that a new word is derived from a more complex form of the same stem.
Typically, a verb is "back formed" from a noun on the grounds of the assumption that a noun ending -er is an agent who does something, so that this something can be signified by a verb. Accordingly, the verb to back form is a back formation of the noun back formation.
A back formation may either be created deliberately or can be due to a faulty morphological analysis: e.g. the word pea was derived from its synonym pease by speakers who thought the latter was a plural.
The identification of back formations involves the difficulty of knowing that a certain noun and the correspondent verb did not enter a language simultaneously but succeeded one another. Speakers of English are not necesssarly aware that the typewriter preceded to typewrite.