A helper agent spawned by a main agent for one bounded job — "research this," "check that" — which reports back and disappears. The main agent keeps the big picture; subagents keep their noses in their own tasks.
Why it matters: subagents solve the context window problem by division of labor: each gets a fresh, focused memory instead of everyone drowning in one shared conversation. Delegation, not just parallelism.
Related: Context window