Sweetened, diluted with water and served with plenty of ice. You can also leave the peel on the lemons (which, confusingly are green not yellow) to make limonada suiça (Swiss lemonade): http://tudogostoso.uol.com.br/receita/15079-limonada-suica.html possibly adding some condensed milk as a sweetener.
Ouch the first sentence immediately knocks its validity as a juice. "Lemonade is a lemon-flavored drink." That's gotta hurt for those of us who squeeze it right outta the lemon.
That's right, so have pity on us poor Brits who are slightly perplexed by this sentence.
I mean store-bought stuff is pretty much tangy sugar-water because I live in America, but I remember a primo juicer in my youth that made the best natural juices, including lemonade!