Clustering is, besides all the other definitions you're seeing here, a table property. A clustered table is a table that has, on disk, been aligned with a certain index[1] - each table can only be clustered to a single index and it essentially means that row retrieval for that specific index is much more efficient, it basically gets you one covering index for free.

1. https://www.postgresql.org/docs/current/sql-cluster.html