Projector
Projectors get their name from Euclidean Geometry but are probably more analogous to a relational database view. The concept is very simple in RIDDL: projectors gather data from entities and other sources, transform that data into a specific record type, and support querying that data arbitrarily.
Projectors transform update events from entities into a data set that can be more easily queried. Projectors have handlers that specify both how to apply updates to the projector's state and satisfy queries against that state. A projector's data is always a duplicate and not the system of record for the data. Typically persistent entities are the system of record.