A merger where a smaller company takes over a larger company. Sometimes this device is used as a means of back door listing.
When a small company notionally buys a bigger one. Although it is the small company doing the buying it is the shareholders of the larger company that end up controlling the enlarged entity. It's a complex process that is mainly used by corporate advisors as an excuse to charge higher fees.
When a small company takes over a larger one, or when the company being taken over is likely to be the major element in the combined business.