If you're talking about stove-top cooking, high heat is generally considered to be anything above a medium setting. Of course, this can vary depending on the type of stove you're using. Gas stoves tend to have higher heat settings than electric stoves, for example.
In general, though, if you're looking to cook something quickly at a high temperature, you'll want to set your stove to a high setting. This will ensure that your food cooks evenly and at the right temperature. Keep in mind that food can continue to cook even after it's been removed from the heat source, so if you're trying to achieve a particular doneness, it's best to err on the side of caution and remove your food before it reaches that point.