The Quick Answer
If you are in American English speaking countries then a jumper is a sleeveless dress meant to be worn over a layer usually a nice shirt or blouse. The dress is normally of medium length and this is meant for warmer seasons.
If you are in a UK English speaking country then what we just described is a pinafore. A jumper in the UK or similar speaking place is what they call a sweater in the USA / Canada etc..
OK so now you know what a jumper is!
Rompers are the similar in the USA and UK English wise. They can cover a few kinds of clothing. For example they can be sexy nightwear or even daywear for outside and in some cases be suitable for summer work clothing.
A romper is one piece with short sleeves or strapless. The bottom is short-like. The bottoms can be bloomer like.
Originally the style was victorian for children to be comfortable but there has been a bit of a craze as celebrity started the trend for womens fashion.
The waist can be belted or cinched up or elasticated otherwise. They are normally quite loose but seductive lingerie could be tighter.
Romper for Women USA
Romper for Women UK
JUMPSUIT (not just for parachutists)
A jumpsuit is a one-piece garment. It can be for men as overalls but less commonly referred to in the modern context as a jumpsuit. It can be casual or somewhat dressy and be appropriate for lunch afternoon or being stylish at home. Usually with a cinched waist and trouser legs of varying style for women.It can be a BODYCON variant too.