A chunk in cognitive psychology is a unit of information.
According to Miller, a chunk can consist of words, numbers, pictures or symbols.
An interesting discovery of the American psychologist is that the size of the 7±2 objects is variable, so it’s possible to group in larger objects (chunks) a certain number of smaller objects, thus increasing the total number of objects remembered.
The fundamental property of a chunk is not its size, but its “familiarity“: i.e. an unusual image, even if simple, cannot be a chunk, on the contrary a very complex sentence (for example an excerpt of a song learned by heart) could be a chunk if it’s very familiar, regardless of its size.