Fan-out
Sending one event to many recipients, such as copying a new post into every follower's feed.
Definition
In feed design, fan-out on write pushes each new post id into every follower's cached feed, so reading a feed is one lookup but posting is expensive for accounts with many followers. Fan-out on read gathers posts from all followed accounts when the feed is opened, which is cheap to write but slow to read. Large systems combine them: push for normal accounts, pull for celebrities.
In practice
A post by an account with 200 followers becomes 200 inserts into Redis feed lists.