Activity is a noun that has a number of similar, but distinct meanings. First and foremost, the term is used to indicate the state of being “active”, and in conjunction with that, and specific movement of energy or liveliness. It also can refer to a specific event, happening, or occurrence in which participation is expected. Activities can be physical in nature, mental, or a combination of the above. Activity can refer to the overall project in a broad sense (i.e., The team was about halfway done with the activity, having only 3 more deliverables remaining until completion), or activity can refer to specific elements within a larger scale project (i.e.,, The project was almost completed, with only one more activity remaining before the deliverables were ready for delivery). Activity can also represent the state of motion among team members who are in the process of working on specific elements of a project (i.e., There was activity among all team members as they rushed to complete the project).
This term is defined in the 3rd and the 4th edition of the PMBOK.