Communication is an essential process in the world of project management (and for that matter the world in which we all live on a day to day basis). It is difficult to master, but essential to make a good effort in achieving. Communication is essentially the effective and complete exchange of information from one person or persons on a project team to other person or persons. Communication on an effective team starts from day one, from the moment a project is assigned, and continues on a daily basis, if not hourly basis, throughout the life cycle of a project. Effective communication means that each and every project team member should be fully apprised of the project status (at least on a macro level) at all times. Some detailed information may not be shared with all team members if they are very individual specific, but the big ticket items should always be shared among the entire team. Communication can also take the form of regular updates to the assigning party as to status of the project and/or specific components of it.
This term is defined in the 3rd edition of the PMBOK but not in the 4th.