Well done Rosie, you got it going! the others have done a great job, but after talkin to you last night I just had to add my bit
Cunards first ship, the Britannia would have deffinatly been a liner, although the term had probally not come into use by then.
Further to the earlier answers, which of course are both correct, a Liner doesnt just have to be built for Transatlantic, but any scheduled 'Line' route, The P&O and Orient Line ships that ran to the Far East and Australasia were all liners, the Arcadia (1) that you sailed round Australia on was a true Liner, The Union Castle ships to Africa were Liners, as were anyship designed to give a long (as opposed to short sea where the ship would be refered to as a Ferry or Packet) sea voyage as a method of transport as opposed to a leisure cruise. All very different ships to most Transatlantic Liners due to the different conditions to be experienced on their routes, but true Ocean Liners (Liners) as well.
In short, Liners for Transport, hence why so rare now we have Jet Planes and Cruise ships just for fun. As has already been pointed out a Liner would be built strong enough to cope with virtually any weather and be designed to keep a schedule, whereas a cruise ship is more lightweight (and cheaper) and would generally have to avoid bad weather, the itinery being ammened, and normally sails only in sunny or scenic places! This is why so many are upset with Cunard for naming the Queen Victoria (which is most deffinatly a cruise ship) as a 'Queen' and even more so naming the nearly identical new Queen Elizabeth after the first Queen Elizabeth which was (arguably) the ultimate Liner, using prestigious Liner names on a cruise ship. Obviously whether that annoys you or not is personal opinion!
As for the older ships. Of the Clippers you refer to, although I don't believe the term liner was in use then, (someone prove me wrong?) some were in effect Liners as they ran scheduled voyages between ports carrying passengers and cargo, although in many cases just cargo in which case they would be cargo liners not passenger liners. The ones which carried passengers, would be the forerunners of the liners. Many ships in the days of sail ran less organised schdules, not one distinct lines, but going as and where demand took them. Those ones would not be Liners at all.
Yes, Titanic was a liner, the largest and grandest of its time, and if by 'recently retired Britannia' you mean the former Royal yacht, then no, thats deffinatly not a liner.
Next?!