A group of keywords and products to use in tactic or analyze in traffic analysis
A segment is a list of keywords and ASINs to use in a tactic or to analyze in traffic analysis.
You can create a segment in 2 ways:
In traffic analysis: Segments let you compare performance per product and keywords
In both cases, when you edit a segment, it is updated everywhere. It means you can create a segment in traffic analysis and add it to a tactic later.
Do not confuse segment and tactic analysis! In traffic analysis, you compare the performance of a segment across all strategies whereas in the overview you compare the performance of the tactic inside a strategy.