For a given cell (minterm) in a k-map, the biggest ‘cluster’ that we can
place it in is known as the prime implicant of that cell.
It is possible for a cell to have multiple prime implicants, multiple clusters
to which it belongs, all of which have the same size.
When a cell belongs only to a single prime implicant, then it is known as an essential prime implicant for that k-map.