Is fixed fee the same as flat fee?

Excuse me, but could you clarify something for me? I've come across the terms "fixed fee" and "flat fee" quite often in relation to financial services, and I'm wondering if they are interchangeable or if there's a subtle distinction between them. Are they essentially the same, meaning a predetermined charge that doesn't vary based on the amount or complexity of the service being provided? Or is there a specific difference in how they're applied or calculated? I'd appreciate your insights on this matter.