In English grammar, conversion is a word-formation

In English grammar, conversion is a word-formation process that assigns an existing word to a different word class, part of speech, or syntactic category. This process is also called zero derivation or a functional shift. The rhetorical term for grammatical conversion is anthimeria. Read to find out how this popular language device can be used and why it came to be.
Why Use Conversion?
But why would one part of speech need to be changed into another? Jean Aitchison, author of Language Change: Progress or Decay? gives examples of how this process is useful. "Consider sentences such as: Henry downed a pint of beer, Melissa went to town and did a buy. English, we note, lacks a simple means of saying 'to do something in one fell swoop.' This may be why the word down can be converted into a verb to mean 'drink down in one gulp,' and the word buy into a noun which, when combined with the verb do, means 'go on a single massive shopping spree.'
This type of fast-moving, thorough activity may represent a change in the pace of life, which is in turn reflected in the language since we increasingly make use of conversions--the conversion of one part of speech into another,"
(Aitchison 1991).
Which Part-of-Speech Came First?
Some words have been functioning as multiple parts-of-speech for so long that their origins are a bit fuzzy. Naturally, for words like this, the question arises: which came first, the noun or the verb? See what author and linguist Barry Blake has to say about this puzzle. "Almost all the examples [of zero conversion] are of shifts between noun, verb, and adjective. In some instances the direction of the shift is clear.
We have had the noun text for a long time, but it has come to be used as a verb only recently with reference to sending messages full of abbreviations via mobile/cell phone. In other instances, we might hesitate to say which part of speech came first, as with plot, for instance. Was it a noun first or was it a verb first?" (Blake 2008).
The Role of Meaning in Conversion
New conversions are still being created in modern English and this will probably always be the case. Language professionals that devote their lives to studying processes such as this one insist that meaning is one of the biggest determinants of whether a conversion would be or is semantically logical—after all, words should not randomly be assigned new syntactic categories. The following excerpt from Approaches to Conversion/Zero-Derivation dives into this topic further.
"Meaning is as crucial to the system of word-classes ... as it is to the recognition of instances of conversion. Even if it were not for the homophonous noun plane 'carpenter's tool,' we would not wish to relate to plane 'smooth a piece of wood' and a plane 'aircraft' by conversion, because their meanings are not sufficiently close. What is a sufficiently close meaning (and how it can be defined) remains an open question.
A slightly dubious example is to bank 'turn an aircraft' and a bank 'side of a hill' which, despite their etymological relatedness, may no longer be close enough semantically for us to wish to say that the same relationship holds between them as between to bridge and a bridge. Somehow, then, we need to operationalize the notion of related in meaning to a sufficient degree to allow us to recognize potential instances of conversion," (Bauer and Hernandez 2005).
Examples of Linguistic Conversion
Linguist conversion can be found in nearly any style of speaking and writing, and some—such as a highly specific noun masquerading as a verb—are much easier to spot than others. This list of examples of conversion will help you understand how it can be used.

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  "Let's not Rumsfeld Afghanistan," (Graham 2009).
  
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  "Boyes spent the night with Mr. Vaughan, and they breakfasted together in the usual way upon bacon and eggs, toast, marmalade and coffee," (Sayers 1928).
  
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  "One writer who went on a tour of New York's Harlem district was shown the place where Adam C. Powell was 'funeralized'. Another letter detailed an American friend's eagerness to see the Prince of Wales 'coronated'. On a flight to Boston, flight attendants promised passengers they would soon 'beverage', but later, because of adverse weather conditions, they said they were 'unable to complete bulverization'. Asked about this trend, one American quipped: 'Any noun can be verbed,'" (Courtney 2008).
  

Conversions in Shakespeare
Even William Shakespeare himself was a fan of this linguistic device and took any opportunity to creatively convert a word. He was a pioneer of normalized conversion, named an "expert" by linguist and author David Crystal. "Shakespeare was the conversion expert. 'I eared her language.' 'He words me.' Some of his conversions seem really daring. Even the name of a person can become a verb. 'Petruchio is Kated.' But all he was doing was tapping into a natural everyday usage that is still with us," (Crystal 2012).