An artifact is anything tangible that gets created. It’s that simple.
Here’s some examples of artifacts:

for any given artifact, we care about the following:
- who’s working on it and the expectations of contribution for all involved
- estimated time of completion
- who the intended audience is
- the intended effect of its delivery to that audience
- once created, reasonable metrics to track the success of the intended effects.
Sufficient information on all of those properties allows anybody to understand the complete lifecycle of a piece of content, and how to effectively plan for future content that’s similar to it or for similar audiences.