An acronym coined from an existing word or name. For example, wiki is a backronym when used for what I know is because wiki is a word whose original meaning is “collaborative website.”
(BACKward acRONYM) A reverse acronym. Instead of turning a multi-word term into letters, such as "central processing unit" into CPU, a given set of letters becomes a multi-word term. For example, TEMPEST was a code name that was turned into "Telecommunications Electronics Material Protected from Emanating Spurious Transmissions." DVD originally meant "digital video disc" but was later changed to "digital versatile disc." See acronym.
The treatment of a word as an acronym even though it is not. For example, ping is a utility used to test a path from one host computer to another across an IP-based network in what is essentially a command to echo the packet from the remote host computer back to the originating host. Ping is a word, not an acronym. However, Dr. David L. Mills reverse-engineered ping into an acronym for packet Internet groper and a great many people believe that was the original meaning. It was not. See also acronym, anacronym, and ping.