Words which are formed from the initial letters of other words.
This definition requires clarification. By 'initial letters', the definition doesn't limit itself just to the single first letter of a word, but to the letters that begin the word. So, 'quango', which is formed from ' quasi non-governmental organization', is an acronym. Conversely, words like 'laser', which is formed from 'light amplification by the stimulated emission of radiation', isn't strictly an acronym as the 'b', 't' and 'o' of 'by', 'the' and 'of' are ignored. In general usage, although not in the piece below, these restrictions are frequently ignored.
Many acronyms are really backronyms.
When looking for the origin of an unexplained word, people sometimes suggest that the word was coined as an acronym of some phrase or other. Occasionally that notion is correct and there are some commonly used words that we might use without necessarily being aware they are acronyms, for example 'gulag' or 'Hamas'. Words that were coined as acronyms form quite a small part of the language. Nevertheless, supposed acronym derivations are the largest source of folk etymologies - those popular fallacies about the origins of words or phrases. With many of these false derivations the word comes first and then some suitably chirpy phrase is invented to match it. These back-formations have been given the intuitive name 'backronyms'. There are many examples and two of the more common, 'posh' and 'golf' (supposedly 'port out, starboard home' and 'gentlemen only, ladies forbidden' respectively), make it onto the Nonsense Nine, my list of popular fallacies. Other backronym examples are:
cop - Constable on patrol
news - North, east, west, south
Those two are at least plausible. Two better known but frankly ludicrous examples are:
fuck - For unlawful carnal knowledge
shit - Ship high in transit
Some backronym coinages make little attempt to feign authenticity and are clearly intended to poke fun; for example, Ford - Fix or repair daily and DOS - Defunct operating system.
Deciding whether an acronym's supposed origin is genuine isn't always so easy. The best place to start is the age of the word. If the word is old, then it probably isn't an acronym. The term 'acronym' itself wasn't coined until the mid 20th century. The earliest known citation of it is from American Notes and Queries, February 1943:
Words made up of the initial letters or syllables of other words... I have seen... called by the name acronym.
Some examples do date from before 1943 but were rare enough beasts in the early 20th century not to have needed a generic name.
The field of computing is now the most prolific source of acronyms - 'gif', 'ascii', 'wysiwyg', 'mpeg', not to mention the names of most programming languages; the list seems endless. Before computers, the military held top spot. Almost all of the earliest known acronyms derive from the armed forces; for example (with the date of their earliest use as an acronym that I know of):
Gestapo - Geheime Staats-Polizei (1934)
Waaf - Women's Auxiliary Air Force (1940)
Radar - Radio detection and ranging (1941)
Prior to these, there were a few military words, which, while being strictly initialisms, come about as close as we can get to being proper acronyms, and many people would accept them as such.
Anzac - Australian and New Zealand Army Corps (1915)
Naafi - Navy, Army, and Air Force Institutes (1927)
So, if someone suggests to you that the name of the 15th century game of golf was coined as an acronym, you might suggest suitable therapy, or possibly a free membership of the Committee Resisting Acronymic Proliferation.