This question drew my attention to pannier here. I just thought that panniers are things that hung on bike. The Wikipedia definition with Google gives:
A pannier is a basket, bag, box, or similar container, carried in pairs either slung over the back of a beast of burden, or attached to a bicycle or motorcycle.
So can I call every bag that hangs even a little on a bicycle a pannier like a handle-bar bag? Or is it essential that it is in pairs? I have panniers-looking side bags but they are not really in pairs rather separated similar bags, I bought them separately. So what are really panniers? Do they need to be attached to one-another or can they be separated? What about if I lost one, is it still a pannier or is it a piece of pannier? When I have a small bag like triangular bag or something else hung from the bike, is it a pannier? Probably not, is the proper idea to think about panniers like a bags on a donkey i.e. rather large bags?