ultrafine, instant dissolving, bar, or castor sugar
Superfine sugar, which has tiny, unifrom crystlals, dissolves well in cold and room-temperature liquids, making it useful for meringues (which weep if there's any undissolved sugar) and in iced tea, lemonade, and cocktails. The tiny granulation can improve the texture of cakes and other butter-and-sugar batters because the crystals' many sharp edges cut into the butter during creaming, forming many air pockets to produce a fine delicate crumb.
Superfine measures like regular granulated sugar.
If you can't find superfine sugar, you can make your own by grinding granulated sugar in a food processor for 30 to 40 seconds.
While shopping for sugar, keep in mind that the sugar industry has not standardized its labels, so stay alert to inconsistencies between brands.
Separate or sift any small lumps before measuring.
Store in a cool dry place in a covered container.