There are two basic definitions for decision point.
The first is on a macro level. It is the latest point in time when a decision must be made within a plan. In many cases, it is advantageous to delay making a final choice until the last possible moment—it keeps options open. But at some point, it becomes too late for the decision to matter.
Think of how a college student might make weekend plans. She may delay committing to any option until as late as possible—a better offer might come along. But if she delays too long, all the tickets are gone, road trips have departed, and games have begun. At that point, it is too late for a decision to matter.
In Lean, this commonly occurs during a kaizenevent. Early in the event, there is a lot of discussion about what to do. But at some point, the team has to make a decision or risk having no time left to make improvements. That is the decision point.
The second definition is the more common one. A decision point is the diamond shape on a process flow chart that indicates a point where a choice must be made, or where status is checked (i.e. “Did the customer order the premium option?”).