"Knockdown" is a texture - check on Google Images using the words "knockdown texture" to get an idea. Getting a perfectly flat ceiling would mean lots of floating, sanding, etc. and could mean some pretty big bucks, but a knockdown texture is cost effective and NOT popcorn. I'm guessing it's the same texture you're getting on your walls, correct?

If so, it'll be nice and cohesive.

Yes, knock-down is a texture they add to a flat ceiling. If they left it flat, then you'd end up seeing the imperfections left by slightly uneven drywall mudding and the texture reduces sheen and glare on a flat surface.

We chose a knock-down ceiling in all our rooms - but we asked for a 4" border of flat ceiling around the edges and then I painted another 4" of white trim at the top of the wall - gives a nice visual effect!

Hi---no it is not a flat/smooth ceiling. Pic for you. Good Luck!