A concert party is a group of people acting in concert in order to take over a target company. Regulators such as the Takeover Panel apply rules applicable to takeover bids to all members of a concert party.
Of particular importance is that the 30% threshold at which a mandatory offer must be made is considered to be reached when a concert party jointly hold 30% of the shares in a company, not when one of them does.
Even entities that are not part of a concert party may find that some rules apply to them: they are required to disclose dealings in the share of the bidder or the target. These "associates" are people who have an interest in the outcome of the bid (other than simply as shareholders) but who are not deliberately acting in concert with the bidder, An example of associates are the directors the target company even when they are not acting in concert with either the bidder or a potential counter-bidder.