What is the exact definition of a dictionary in the concept of linear algebra and computer science? I am reading this paper for my master thesis and trying to understand every concept in it. The following sentence confuses me as to what is the exact definition of a dictionary in this concept except for the fact that it holds the atoms as its columns?


  Recall we refer to the columns of the matrix $A$ as atoms that are later used as building blocks in the construction of $b$. Such a matrix that holds the atoms as its columnsis referred to as a dictionary[...].
Yes, as it says, a "dictionary" is just the word they're using for a matrix with entries of a particular type that's used for a particular purpose. They're drfining the term "dictionary" in the text you quote.