A man-in-the-middle attack (MITM, sometimes called a "bucket brigade attack") is an attack where the attacker secretly relays and possibly alters the communication between two parties who believe they are directly communicating with each other.
One example of man-in-the-middle attacks is active eavesdropping, in which the attacker makes independent connections with the victims and relays messages between them to make them believe they are talking directly to each other over a private connection, when in fact the entire conversation is controlled by the attacker. The attacker must be able to intercept all relevant messages passing between the two victims and inject new ones. This is straightforward in many circumstances; for example, an attacker within reception range of wireless access point can insert himself as a man-in-the-middle.
Source: Wikipedia, May 2017