Hirame (hee-rah-meh) is the Japanese term for any type of flat, bottom living horizontally-oriented fish (the word meaning literally “flatfish”), but in cuisine (sushi) it is primarily (but not limited to being) used for Fluke, which is actually another word in the US for Summer flounder. There are actually many different types of flounder, but for culinary purposes there are two that are most commonly found in a restaurant, Summer Flounder and Winter Flounder (also called Blackjack in some areas). Winter Flounder is actually called karei in Japan. If a person were to order hirame in western countries, they would generally be served either Fluke, or in some cases it turns out, Halibut.
As with most nigiri-zushi, one order is generally a pair, and in the case of hirame, the meat is taken from the area around the fish’s fin and is called the engawa, which means ‘porch.’ The size of the neta (or tane) can be a clear indicator of the type of fish that is being served as hirmae (Fluke) pieces are often so small that a few (usually two) are needed to top the rice (shari). Ohyo (Halibut) will be a single piece of fish on top of the shari. Hirame is also more common in winter months, when the fish is at its leanest, and in Japan, this is considered the best time to serve hirame. Summer hirame has a tendency to be fattier and mushy, the antithesis of the light, clean flavor and firm texture of winter hirame. Halibut, on the other hand, is served year round with consistency.
While both hirame and ohyo are excellent choices, lighter fare with a sweet, subtle flavor, they are most certainly different meats. It is doubtful anyone would be unhappy being served either, they are similar enough, however it is always good to know what you are getting, as an educated consumer will know what to specifically ask for upon subsequent visits to any sushi-ya.