how do u define a senior developer and mid-level developers?

Your mid levels should be bashing out great code, coaching the juniors and should be the bedrock of your team.

Your seniors should be taking ownership of projects in a lot of ways, coaching the mids and juniors etc

A lot of it is around personality types I think. You need to be an authority in the team to be a senior, to my mind.

i thought it was all just about the level of knowledge. :/
I've met some senior devs, some are which I approve are senior and some are not so good which seems more to be a mid-level according to their level of knwldge. thus wanted to get others opinion on their definition of senior dev.

Knowledge is great but if you aren't a good communicator or you aren't a patient person, how do you better your colleagues? How do you help to permeate that knowledge and improve the way your team works? And how do you explain to your PO that they can't have the thing their want because they've requested it mid sprint? And what happens if your lead is on holiday? You need to be able to take control of a situation and if you can't, you're just writing great code. There's a reason software houses all encourage employees to host coding dojos, lightning talks and brown bag sessions. It's because it's generally understand that a long with tech debt, we've got this idea of knowledge debt where one Dev knows everything and everyone else depends on them. It's how you get burnt out, unhappy devs!

/rant