In the UK this would most commonly be called a hall assuming it has doors, arches or similar leading into other rooms. A hall is normally a long, often narrow, room with several rooms opening off it (corridor is rarely used inside the home) but the minimal hall is probably the one I had in my old house: about 1 metre square, with the front door living room door and stairs leading from it.
As to whether it's considered a room, that's variable. When thinking about decorating, for example, yes - "which room are you going to decorate next?", "The hall". But in terms of counting rooms to describe the size of the house, generally a hall isn't a room. Wider examples may be an exception to this - some are big enough to use a corner as an office or seating area.