The definition
A supplement is an extra charge added to a set price when a guest chooses a more expensive option within a course.
It lets a restaurant keep one headline price on a prix fixe or tasting menu while still offering a dish that would break that price: a dry-aged steak instead of the chicken, shaved truffle on the risotto, a half dozen oysters in place of the soup.
How you hear it
The server does the arithmetic for the guest. If they have to, the menu probably did not make it clear enough.
Where it shows up on the menu or the check
On a printed prix fixe, the supplement sits next to the one dish it applies to, written as “supplement 15”, “+15”, or “add 15”, in smaller type than the dinner price. The per-person price for the whole meal stays at the top where everyone reads it first.
On the check, the supplement is its own line or a modifier on the course, added to the per-person price for that guest only. On a table of four where one person takes the steak, only that seat pays it.
Supplements also show up outside set menus: a truffle add-on, an extra shot, a larger pour. On an à la carte menu, most restaurants call those add-ons or upcharges instead.
What it means for prices and printing
The point of a supplement is to protect the headline price. If three of five mains carry one, the headline price is not really the price; raise it and drop the supplements.
Price a supplement from the difference in plate cost, not from what sounds right. If the steak costs the kitchen twelve dollars more than the chicken, a fifteen dollar supplement barely covers it once you add the time at the grill.
In Gaslamp a supplement is its own line: “Strip steak supplement” at 15, in the same section as the course it upgrades, or in a Supplements section of its own right after the mains. Print the rule once in the note under the heading, for example “Supplements are per guest and added to the dinner price.”
Where it comes from
“Supplement” comes from Latin supplementum, from supplere, “to fill up”; English has used the noun since the late 1300s for something added to make a thing more complete, according to the Online Etymology Dictionary. We could not find a reliable source for when menus started using it for an added charge.
Go deeper: Prix fixe and tasting menus