Self-Doubt
Where these two overlap, and where they differ.
Imposter syndrome usually gets used as shorthand for a specific version of self-doubt, feeling like a fraud in a role you're objectively qualified for, paired with a fear of being "found out." Self-doubt is the broader category, and it doesn't always come with that specific fear.
Both involve a gap between external evidence of competence and internal belief. Both tend to get reinforced by achievement, oddly, because success just raises the stakes of being "found out" or resets the bar for what counts as good enough.
Imposter syndrome is usually tied specifically to competence and performance, often in professional or academic settings. Self-doubt is broader; it can show up in relationships, decision-making, parenting, or just a general sense of not trusting your own judgment, with no achievement context at all. Someone can have significant self-doubt without ever describing themselves as feeling like an imposter.
Naming which one you're dealing with changes where we focus. Imposter syndrome tends to bring up internalized standards and comparison. Broader self-doubt usually means going further back, into where the belief that you can't trust yourself first got built.