It's a funny term, but it's the method that is uses a piece of metal (like tin or aluminum) to melt two pieces of metal together.
I love how many ways I've seen all of us (myself included at one time) have called it something other than *solder*. I had a very "oh! duh!" moment months ago when I realized I was spelling it wrong for quite some time. haha. Whoops!
How have you always spelled it/thought it was spelled?