Wiktionary
n. (context economics English) The integrating of identical stages in the production or marketing process under the ownership or control of a single management organization.
WordNet
n. absorption into a single firm of several firms involved in the same level of production and sharing resources at that level [syn: horizontal combination]
Wikipedia
Horizontal integration is the process of a company increasing production of goods or services at the same part of the supply chain. A company may do this via internal expansion, acquisition or merger.
The process can lead to monopoly if a company captures the vast majority of the market for that good or service.
Horizontal integration is orthogonal to vertical integration, where companies integrate multiple stages of production of a small number of production units.