Finally, a definitive answer.

I thought it was full because 50% water and 50% air and therefore 100% filled.

Shouldn't we be more specific? What is the glass filled with? Is it water? Piss? Baby blood? Did the blood come FROM a baby? Or is it going TO a baby? I feel like that sort of changes the outlook...

That's what I've been saying all along

"Sorry for ruin your philosophy"?
.
Dat grammer tho

Or my personal favorite
Half empty or half full... doesn't matter. There's room for vodka..

That's how I always looked at it.

Thank you, I've been saying this for years.

Thanks, like I say: "it all depends on context."

finally, someone gets it from my point of view

The completely neutral answer:
The glass contains half its capacity for liquid.
The glass is completely filled with fluid, as air is also a fluid.