When my children were infants, they wore diapers. Every so often I would splurge on a box of the newfangled, expensive, disposable kind. Now when people say “diaper,” they mean the disposable kind and would call the kind I used “cloth diapers.”
Earlier than that, back when he mowed the grass, my big brother longed for an “electric mower.” Now the kind of mower he called a “lawnmower” is called a “push lawnmower.”
“Cloth diaper” and “push lawnmower” are retronyms.
The term retronym came into the language in 1980 when William Safire credited Frank Mankiewicz, president of National Public Radio, with its first use.
Here’s the OED definition of retronym:
A neologism created for an existing object or concept because the exact meaning of the original term used for it has become ambiguous (usually as a result of a new development, technological advance, etc.). A retronym typically consists of the original term combined with a modifying word.
Here’s the definition from the Merriam-Webster Collegiate Dictionary:
a term consisting of a noun and a modifier which specifies the original meaning of the noun <"film camera" is a retronym>
Although the term is new, the practice is old.
For example, before the invention of the breech-loading rifle, the most common way to load a gun was by way of the muzzle. When the new “breechloader” became common, the retronym muzzleloader was born.
Same thing with “World War I.” Before the invasion of Poland in 1939, the global war that took place between 1914 and 1918 was known as “The Great War,” or the “14/18 War.” There had to be a WWII before there could be a WWI. World War I is a retronym.
Ditto “silent movies.” Before the invention of “talkies,” movies without sound were just movies.
Retronyms are created at a dizzying pace as words that once meant just one thing require more and more modifiers:
coffee – regular, decaf, specialty?
mail – email or snail?
mother – biological, adoptive, or surrogate?
oven – microwave, conventional, convection?
restaurant – sit-down, take-out, fast-food?
soap – bar, liquid, body wash, soft, gel?
telephone – landline, cell, wireless?
television – analog, digital, or HD?
Historical novelists aren’t the only ones who have to worry about anachronistic vocabulary.