horizontal - uniform; producing or based on uniformity. Chiefly U.S. Times 22 May (Margarine Suppl.) p. viii, The Council of the European Community has already issued a directive in connexion with the use of colour additives. This follows what is known as the 'horizontal' system, meaning that it is of application to all food products., in industry: horizontal combination, integration, merger, an industrial merger of firms engaged in the same stages or types of manufacture; so horizontal combine, the organization resulting from such a merger