What is a threshold condition in a learning environment?

A threshold condition in architecture is a physical point at which a change between two spaces can be acknowledged. But what is the definition of a threshold condition in time? Particularly what is a threshold condition for a learning environment?

Every session of a course must start and end at a certain time. So we can call the beginning and end of a course session threshold conditions... there... 2 thresholds.

... beginning

{ stuff happens }

... end

Perhaps there are more specific thresholds that might be identified?

For example,

+ points at which someone asks a question that changes the flow of conversation in a seminar
+ each new presentation in a sequence of presentations
+ 2 portions of a lecture (lecture & Q/A)
+ each new comment in a seminar
+ different concepts presented in a workshop

...