In IT environment we hear the term (man in the middle attack), what is its definition?

I fully agree with MR . Sayed Albeez.

Man-in-the-middle attack is a form of active eavesdropping in which the attacker makes independent connections with the victims and relays messages between them, making them believe that 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 messages going between the two victims and inject new ones.