Hay, Donald A.
Morris, Derek J.
1991
Industrial Economics and Organization: Theory and Evidence
Oxford University Press; Oxford

496
Definition-horizontal merger:
“A horizontal merger is one in which both firms are in the same product market. A vertical merger is one in which a firm acquires either a supplier or a customer firm. If there is no horizontal or vertical relation between two merging firms, then it is defined as a conglomerate merger. In practice, many mergers between diversified companies include elements of two or even all three of these classifications.”