From IPAPI

Activity in the context of process refers to the individual units of work that when combined, often in a specific order, make up the overall work of a process. How activities are defined varies widely, as does the operating definition of activity.

Activities have a natural separation based on characteristics like the purpose of the activity, who performs it, where it is performed, why it is performed, and how it impacts people (internal and external).

IPAPI Definition

Activity – An activity is the basic unit of work of a process. Activities follow natural lines of separation based on the interim outcome or purpose of the activity within a process.

Overview

The term “activity” is commonly employed in reference models, standards, and practices related to process although there is no common definition in use. Some practices identify specific activities for process such as start, stop, task, sub-process, and stop activities.

Because activities are the basic unit of work of a process, each activity should be descriptive of the work it represents. It is extremely important for us to recognize that the natural separation of activities within a process already exists and that people can readily recognize these breaks without reference models, standards or prescriptive practices. Put a group of people in a room and tell them to identify the activities of a process they are either familiar with or have process documentation on and they will have little difficulty in doing so.

This natural separation of activities within a process is important. Using this natural separation provides a common ground of shared understanding by the people in the organization and leads to enhanced improvement of processes.