Altering or adding to a prior word or term term that must be further defined in the light of later developments or technical innovation.
Example: No one called "World War One" that until there was a "World War Two" with which to contrast it. The going term during the 1914-1918 war and up to 1939 was "The Great War."
Other employment of term retronym:
Telephone becomes "rotary-dial phone" to distinguish it from the push-button phones that became widespread in the 1970s and early 1980s (although rotary-dial phones still work if all you want to do is place a call and don't need to access features like querying a bank account balance).
Similarly, telephone also becomes "corded phone" to distinguish the traditional hard-wired telephone from those that are wireless in some way, such as cordless phones.
"Regular" coffee to distinguish it from decaffeinated coffee; some people say "caffeinated" coffee but strictly speaking this is a grammatical back-formation, not a retronym, because "to caffeinate" would mean to ADD caffeine to traditional coffee.
Note, though, that Coca-Cola is a "caffeinated" or "caffeine-containing" soft drink in its usual red-can form. Now that there is a Caffeine-Free Coca-Cola "caffeinated" could find use as a retronym for "the real thing."
"Manual" or "standard" or "stick" transmission on a car, none of which terms was necessary before automatic transmissions on cars became widespread and assumed to be the norm.
And, of course, "acoustic" guitar.