For us, an epic is a unit of related work which is usually sized for completion in under a quarter or two. Many epics may be completed to get through an initiative. Ex: initiative may be “make a new service to replace section $FOO of a monolith.” If this is able to be done in a quarter or so, then it is the epic. If it is larger, we may break it up into milestone epics like prototype, beta release, full release, and clean up. Like you say epics will be composed of stories, tasks, and spikes (investigations).