Longman Dictionary of Contemporary English
Wiktionary
n. food with little or no nutritional value.
WordNet
n. food that tastes good but is high in calories having little nutritional value
Wikipedia
Junk food is a pejorative term for cheap food containing high levels of calories from sugar or fat with little fiber, protein, vitamins or minerals. Use of the term implies that a particular food has little " nutritional value" and contains excessive fat, sugar, salt, and calories. Junk food can also refer to high protein food like meat prepared with saturated fat -which some believe may be unhealthy, although some studies have shown no correlation between saturated fat and cardiovascular diseases; many hamburger outlets, fried chicken outlets and the like supply food considered as junk food.
Despite being labeled as "junk," such foods usually do not pose any immediate health concerns and are generally safe when integrated into a well balanced diet. However, concerns about the negative health effects resulting from the consumption of a "junk food"-heavy diet, especially obesity, have resulted in public health awareness campaigns, and restrictions on advertising and sale in several countries.
Usage examples of "junk food".
And while we consume mass quantities of junk food at Claudia's, Dawn usually munches on an apple.
I'd go back to my apartment, change into my jammies, and curl up on my couch, where I'd watch junk TV while I ate my junk food.
I couldn't believe what we were doing and just to prove a point about lobster which is really so overrated as a food, I'm sorry to go on about lobsters so much, I'll try and stop in a minute, but they've been on my mind so much for the months I've been in this tank, can you imagine what it's like to be stuck in a ship with the same guys for months eating junk food when all one guy will talk about is lobster and then spend six months floating by yourself in a tank thinking about it.
In between taking calls and setting up appointments we came up with a million ideas for booths, some of them completely whacked out (decorative trees made of junk food - that was Claudia's), some of them more reasonable.