Meaning of the adjacency matrix product Let $A$ be an adjacency matrix of a directed graph. What's the meaning of the $(i,j)-$entry of the matrix $((A^T)^{7} \cdot (A^{7}))$ ? 

My initial interpretation is that $(i,j)$ of this matrix is zero whenever nodes $i$ and $j$ have no 7-length in-coming paths from a common source. Is that right? Any attention is appreciated!
This is answered on  Math.SE; see also  MathWorld . 

Briefly,the $(i,j)$-th entry in $A^n$ gives the number of directed walks from vertex $i$ to $j$ that have length $n$. Also, given another adjacency matrix $B$, this entry in the product $(AB)$ gives the number of directed walks from vertex $i$ to $j$ , that walk first along an edge of the first graph and then along an edge of the second.Putting these two together would give you an interpretation to the product of 7th powers in your question.