[nend]

Breakthrough:

1. a sudden advance especially in knowledge or technique

2. an act or instance of moving through or beyond an obstacle

Milestone:

1. an action or event marking a significant change or stage in development.

I'm failing to see why labeling it one or the other even matters.

[felipellrocha]

Some people think semantics is a good spend of time... These same people tend to amount to nothing, so I just ignore nowadays.