What is a bitcoin physically?

I've read a few books about Bitcoin, and I have a vague idea about Bitcoin. I understand its power, I understand that there will be fewer and fewer of them, its decentralization, etc., and so far, I think that a Bitcoin is something like a digital property title within a registry that is the block chain.

But I can't quite imagine what a Bitcoin is physically. That is, if I have a Bitcoin, what I really have is the certainty that in the block chain there is a note that says "This pseudonym has a Bitcoin."

Am I right? Could someone with a better capacity for clarification or divulgativwle give me a better explanation?