Is "odom" frame in this wiki mean a world coordinate?

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• /odom is the standardized name of the world frame of the odometry component (robot movement is continuous but the frame drifts)
• /map is the standardized name of the world frame of the localization component (do not drift, but movement is not continuous)

You may have none, one or both depending on what components are running. robot_pose_ekf is the traditional package for providing odometry based on the integration of several sensors output (wheel odometry, IMU and a third optional source such as visual odometry). You also have viso2_ros which is dedicated to visual odometry.

An example of localization without odometry would be, for instance, a motion capture system (motion_analysis_mocap) or the ground truth of a robotics simulator (gazebo).

You can see the interest of tf here as any of these setup can be used and it is totally transparent for the navigation stack as long as the frames are defined.

Here in this example robot-base location (via transform publish from /odom to /base_link) is computed w.r.t. /odom frame. The /odom frame is fixed frame so you can assume it is acting as world frame. But a world frame could be completely different from /odom or /map.