Yellowtail is the English translation for “hamachi “in the Japanese restaurants. However the Japanese have tons of names for fish, and “hamachi” only means the young yellow tail.
Variants of Yellowtails incluide:
Hiramasa: Yellowtail amberjack
Inada (very young yellowtail)
Buri(old yellowtail)
Yellowtail is a kind of shiromi(white meat fish) and is one of the milder tasting fish among all sashimi fish and you should probably eat it first in a sushi or sashimi meal. That is why some in hamachi handrolls and hamachi tekka rolls, green onion is mixed with the sushi fish to enhance the flavor of the fish.
Sadly enough, since almost all of the fish in US (except Tuna, I think) are flash frozen*. Hamachi will taste even blender without being ultra fresh. In the worst cases, it would taste like watery and you can feel the “iciness” of the flash freeze process.
*According to New Hampshire's WMUR television station, fish that is suitable for eating raw must be frozen for seven days at negative 4 degrees Fahrenheit, or flash frozen for 15 hours at negative 31 degrees. This is an effort to kill all the parasites in the fish. However, the sashimi is tarnished in the process. I have heard that nowadays they have a better flash freezing technology so it can preserve the fish better in the flash freezing process but I have yet to try some good ones.