Reentrant Callback
A reentrant callback is a callback function that satisfies the reentrancy property: no mutable shared state, safe to invoke concurrently or nested. First saw this in ROS 2.
In ROS 2 it's a scheduling policy
A Reentrant Callback Group tells the executor it may run callbacks in that group in parallel, including the same callback concurrently with itself. The group is a permission, not a property. You’re promising the callbacks are actually reentrant; if they aren’t, you’ve created a data race.
Compare with the default Mutually Exclusive Callback Group, which serializes callbacks in the group. See ROS Callback Group for when to pick which.