A project plan, according to the Project Management Body of Knowledge

Project Management Body of Knowledge

A Guide to the Project Management Body of Knowledge is a book which presents a set of standard terminology and guidelines for project management. The Fourth Edition was recognized by the American National Standards Institute as an American National Standard...

, is

"...a formal, approved document used to guide both project execution and project control. The primary uses of the project plan are to document planning assumptions and decisions, facilitate communication among stakeholders, and document approved scope, cost, and schedule baselines. A project plan may be summarized or detailed."

PRINCE2

PRINCE2

PRojects IN Controlled Environments 2 is a structured project management method endorsed by the UK government as the project management standard for public projects. The methodology encompasses the management, control and organisation of a project...

 defines:

"...a statement of how and when a project's objectives are to be achieved, by showing the major products, milestones, activities and resources

Resource (project management)

In project management terminology, resources are required to carry out the project tasks. They can be people, equipment, facilities, funding, or anything else capable of definition required for the completion of a project activity. The lack of a resource will therefore be a constraint on the...

 required on the project."

In some industries, particularly information technology

Information technology

Information technology is the acquisition, processing, storage and dissemination of vocal, pictorial, textual and numerical information by a microelectronics-based combination of computing and telecommunications...

, the term "project plan" can refer to a Gantt chart

Gantt chart

A Gantt chart is a type of bar chart that illustrates a project schedule. Gantt charts illustrate the start and finish dates of the terminal elements and summary elements of a project. Terminal elements and summary elements comprise the work breakdown structure of the project. Some Gantt charts...

 or other document that shows project activities along a timeline. While common, this use is inaccurate. These types of documents are more accurately described as "project schedules

Schedule (project management)

In project management, a schedule consists of a list of a project's terminal elements with intended start and finish dates. Terminal elements are the lowest element in a schedule, which is not further subdivided...

" and are only one component of a true project plan.

At a minimum, a project plan answers basic questions about the project:

• Why? - What is the problem or value proposition addressed by the project? Why is it being sponsored?
• What? - What is the work that will be performed on the project? What are the major products/deliverable
  Deliverable
  Deliverable is a term used in project management to describe a tangible or intangible object produced as a result of the project that is intended to be delivered to a customer . A deliverable could be a report, a document, a server upgrade or any other building block of an overall project.A...
  
  s?
• Who? - Who will be involved and what will be their responsibilities
  Accountability
  Accountability is a concept in ethics and governance with several meanings. It is often used synonymously with such concepts as responsibility, answerability, blameworthiness, liability, and other terms associated with the expectation of account-giving...
  
   within the project? How will they be organized?
• When? - What is the project timeline and when will particularly meaningful points, referred to as milestones, be complete?

To be a complete project plan according to industry standards such as the PMBOK or PRINCE2, the project plan must also describe the execution, management and control of the project. This information can be provided by referencing other documents that will be produced, such as a Procurement

Procurement

Procurement is the acquisition of goods or services. It is favourable that the goods/services are appropriate and that they are procured at the best possible cost to meet the needs of the purchaser in terms of quality and quantity, time, and location...

Plan or Construction Plan, or it may be detailed in the project plan itself.