Per Dave Wondrich (on a different message board):
...There are two ways of looking at drink families in play here, the historical and the mixological. While these tend to overlap quite a bit, the alignment is never perfect, leading to off-kilter debates.....
Historically, there's the cocktail/cock-tail. This evolved over time, roughly thus:
ca. 1800: spirits, water, sugar, bitters; stirred
ca. 1840: spirits, syrup or sugar + splash of water, bitters, ice; rolled or stirred; served with ice
ca. 1843: option of adding absinthe
ca. 1860: spirits, syrup or sugar + splash of water, bitters, ice, lemon twist; shaken or stirred; served up or with ice
+ option of adding curacao
+ " " " " " sugar rim
+ " " " " " splash of lemon juice
These three options prove, for some, to be a trigger-point, forcing the name of the drink to be qualified as "fancy" or changed to "crusta." But for the general run of drinkers they don't bring the drink to the threshold of name-change. In other words, they're modifications, not alterations.
Then:
ca. 1870: + option of adding vermouth
But this addition of vermouth proves to be a trigger-point, forcing the creation of an "Old-Fashioned" cocktail category, where the purists get their say. This is hinted at in print in the 1860s, discussed by 1880 and first appears in a cocktail book in 1888 (Theodore Proulx's Bartender's Manual). This is analogous to what happened to the Gin Martini in the 1980s, or the Mint Julep after the Civil War, where the most reductive, narrow definition of the drink was held up as the original standard, whereas the original drink being evoked allowed much more latitude, as it had yet to become a marker of identity or a subject of ideology.
This "Old-Fashioned" style cocktail then undergoes evolution of its own: by 1916, the garnish has gotten a little more fancy. By 1933, the drink has absorbed the fruit-garnished Whiskey Toddy, making the muddled Old-Fashioned of 1940s-1960s fame. (Then, of course, come the new purists, and . . . .)
There's a period, roughly 1870-1905, when the original cocktail and the Old-Fashioned overlap; when you could order a "whiskey Cocktail" in a bar and expect in most instances to get a shaken blend of whiskey (or gin, or brandy), syrup, bitters, perhaps with a splash of curacao or even maraschino and a dash of absinthe, strained and served up with a twist. You might have to talk to your bartender a bit first, though.
As for the mixological Old-Fashioned:
The original cocktail can also be defined as a composition of spirits, sweetener, bittering agent, diluent (that, as Wikipedia informs me, is the correct word for 'diluting agent') and optional citrus oil float.
Each of these components can, of course, be viewed prismatically, so to speak: the spirit element can be unitary--whiskey--or composite--tequila and mezcal. The same goes for any of the other components. Ingredients can easily do double duty, e.g., functioning as both bittering agent and sweetener.