Activity is the aggregation of tasks needed to deliver a definable part of a sub assembly or service. An example is the milling of a part that will become part of a sub assembly. Here the raw material will need to be heat treated, then milled, then degreased, then polished, then tested for tolerance.
These tasks form a definable outcome or part of a sub assembly. In a service business (insurance), an example is the claim review, which may be part of the claim adjudication subprocess, which in turn may be part of the line of business management process.
Activities can aggregate to form scenarios. These are groups of activities and their tasks that are always executed in certain events or in response to specific needs — such as customer registration or on‐boarding in a banking wealth management line of business.
Source: Guide to the Business Process Management Body of Knowledge – ABPMP BPM CBOK V3.0.