How do you define a junior/senior dev?

I've been a professional developer for about 5/6 years now.

Would taking a "junior" role be a step down? Or does the term not really matter?

Different companies define it in different ways, but if you've got 5/6 years experience then I'd say it's almost certainly a step down. You should really be starting to shoot for the senior roles above the junior ones at that point.

It really just depends. Obviously, the terms "junior/senior" indicate level of experience, but you could be experienced with one form of development (server/software/web). You could be experienced with one form of development over another, and it's up to your employer to decide which level of role you'll be in based on your experience with what you're working on. If you're changing development roles and you're less experienced with what you'll be working with, you may be a junior role but you won't necessarily be "a step down". Depends on your salary and level of decision-making.

Senior is junior + lots of experience. Don't sell yourself short.