Segment
A rule-defined slice of your contacts that re-evaluates itself, rather than a list you maintain by hand
A segment is a set of contacts defined by a rule instead of by membership: everyone in Germany who has opened something in the last ninety days, everyone on a trial that ends this week, everyone who has never clicked anything.
The rule is the definition, so the membership is always current. Nobody has to remember to add or remove anyone, which is the difference between a segment and a static list.
Why it matters beyond convenience
Sending less mail to more relevant people beats sending more mail to everyone, and it does so on the metric that decides whether your mail arrives at all. Engagement is a positive reputation signal; a large audience that never opens anything is a negative one, even with no complaints.
Excluding your unengaged contacts from a broadcast is usually the single cheapest deliverability improvement available.
In Sendly
Segments are built from contact fields, custom fields, list membership and tracked behaviour, and are re-evaluated rather than snapshotted. A campaign can be aimed at one, a workflow can be triggered by entry into one, and the API can page through a segment's contacts with cursor pagination — which matters when the answer is a few million rows.