Doesn't matter. Either are the wrong terminology for what their true intention is. I just used what the other guy used since I have heard them referred to either way long before. I didn't see the need to correct a common misconception since everyone knew what was being discussed.

But since you brought it up, the holes are actually core holes which are utilized for supporting the coolant passage core(s) during the casting process and for getting the sand out of the water jackets after casting the block. The plugs are there to seal the holes back up so the proper name for them would be core plugs. And since they won't necessarily save a block if it freezes, calling them freeze or frost plugs is inaccurate.

 

Yep, we've been down this road before and for those needing more info on the reason for those plugs, they can read these pages:
Engine Freeze Plugs or Expansion Plugs

Core Plugs

 

freeze plugs,sure whatever you want to call them i am canadian so cut me some slack

and actually they are casting plugs,swartlkk has got it right