What Actually Separates a Senior Developer from a Junior?

The biggest difference: if you hand a senior dev something, they can take it from start to finish. A junior usually needs help along the way. Seniors not only ship, they keep everyone in the loop and teach as they go so juniors can level up. Context matters too. A dev can feel senior in one codebase and junior in another. But the core question stays the same: can they handle the whole thing on their own, or do they need someone to pull them through?