When is a Dreadnought a Dreadnought

To all and sundry,

Just something to throw around, but I was wondering in relation to
mass and ship class but what makes a ship a Dreadnought i.e. any 
Capital ship can be a Dreadnought, or must it start at a certain mass, 
and what makes a Carrier (other than just having lots of fighter bays)
I know it sounds like a strange question but I just needed some outside
input to clear up my mind.	

tks 

Paul Frigo
Paulf@defcen.gov.au
Canberra
Australia