The first definition is the most usual one. Jumper is a very common word to describe clothing in British English, and is pretty well a synonym of sweater or pullover. It's made out of wool or similar material, stretches from your neck to your waist, and usually has long sleeves. You put it on over your head (or in other words, it doesn't open up the front - that's what we'd call a cardigan). I believe that the second definition is an American one, but fairly unusual. This would be more commonly described as a pinafore or pinafore dress.

This would depend on where you are. I live in the Southern US (Texas) and we don't use Jumper to describe clothing at all. From conversations with other people and reading in the past I can tell you that if jumper is used in the context of adults it tends to be the first definition (similar to a sweatshirt or hoodie) and if discussing a young girl, it is more likely to be the second. Hope this helps.

Speaking from a UK point of view, I would say that definition #1 is correct. Also sometimes referred to as a 'Wooly Jumper'. I can't say I've ever come across definition #2 in the UK

The second definition is definitely an American understanding of the word "jumper." I have never heard it used to describe a sweater or pullover outside of British TV or movies. I suggest you choose the definition that will be most useful -- if you will be traveling in the UK, definitely the first. If you will be visiting the US, definitely the second. For what it's worth, I think only little girls in the US actually wear anything that I would call a jumper -- what Su.Ki. described as a "pinafore" (a word that I think is understood, but definitely not a word that is commonly used in the part of the US where I live). I wouldn't consider such a dress to be adult clothing.