Segment

A segment is a named subset of users, sessions or events defined by a shared property or shared behaviour, through which any report can be viewed.

Every segment is fixed by three choices: what it is built on, which can be a user property, event behaviour or membership of a cohort; whether it is live or static, meaning membership is recomputed on every query or the list is frozen when it is created; and the unit of analysis, which is users, sessions or events. That unit causes the largest discrepancies; a mobile segment counted by users and by sessions returns two different numbers, because one person can arrive on both a phone and a desktop. Two naming collisions muddy search results: Segment is also the name of a customer data platform product, and in GA4 segments exist only in Explorations while the equivalent control in standard reports is called a comparison. The common analytical error is carving a segment out of the outcome itself; a segment of users who purchased will always look more engaged and says nothing about cause.