A phrase based on the concept of "acting in concert", which is defined in the City Code on Takeovers and Mergers as active co-operation by agreement or understanding (whether formal or informal) to obtain or consolidate control of a company. The definition states that certain classes of person will be deemed to be acting in concert with each other unless the contrary is established (for example, companies with their directors and subsidiaries).
Practical Law Dictionary. Glossary of UK, US and international legal terms. www.practicallaw.com. 2010.