What is the threshold for sparse matrices? Is it a matrix that contain less than 50% 0's?
You can't locate a definition because there isn't one.  "Sparse" is whatever relation makes a different algorithm more efficient.  It may be a particular proportion of elements; it may be a function of the matrix side (e.g. n element in a nxn matrix); it may require zero rows or diagonals.
It depends critically on how you plan to alter your handling of a "sparse" matrix.
When we learned the basics of sparse representations, we used a heuristic of 10% non-zero elements.  However, that was a particular family of OS data storage and retrieval.
Does a "sparse matrix" mean that it contains *more than 50% 0's?
I can't seem to locate that information.
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