Merger: A merger takes place whenever two companies unite to form a single company. This is very alike to an acquisition or takeover, apart from that the existing stock-holders of both companies comprised retain a shared interest in the latest corporation. By contrast, in an acquisition one company bought a bulk of a second company's stock, making an uneven balance of ownership in the latest combined company.
The whole merger process is generally kept secret from the general public, and frequently from the majority of the employees at the included companies. As the majority of attempts do not do well, and the majority is kept secret, it is hard to estimate how many potential mergers take place in a given year. This is likely that the number is very high, though, given the total of successful ones and their desirability for numerous companies.