Define Activities process falls under the Planning Process Group in the Schedule Management Knowledge Area.

The main aim of the Define Activities process is to divide the work into activities. This division will help in estimating the time required to complete these activities.
Question. Where do you get this ‘work’ from that you are trying to break down into activities?
Remember all the hard work we did in the Scope Management knowledge area to come up with the WBS? Exactly! That is where the project’s Work Packages dwell. These Work Packages are decomposed further into Activities. This is primarily done because of the fact that Work Packages are larger and thus difficult to estimate but breaking them down into Activities is going to help with estimation.

You might recall this from our brief discussion about the Two Types of Work Breakdown Structures (WBS)
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