Also called: PlanState, execution node, plan node at runtime
In plain English
When the executor runs a plan, each plan node becomes a live executor node (a PlanState) that holds the running state for that step — its position, its buffers, its children. Each one knows how to return the next tuple when asked.
Why it matters
The whole executor is a tree of these nodes pulled from the top, one tuple at a time. The per-node numbers in EXPLAIN ANALYZE — actual rows, loops, timing — are measurements of individual executor nodes, so reading them is reading what each node actually did.