in reference to a menu of items priced and ordered separately, i.e. the usual operation of restaurants. This is in contrast to a table d'hôte, at which a menu with limited or no choice is served at a fixed price.
to order an item from the menu on its own, e.g. a steak without the potatoes and vegetables is steak à la carte
also in reference to pricing models, e.g. à la carte pricing