What's a Sweater or Pullover ?
A sweater, also called a jumper in British English, is a piece of clothing, typically with long sleeves, made of knitted or crocheted material that covers the upper part of the body. When sleeveless, the garment is often called a slipover or sweater vest.
Sweaters are worn by adults and children of all genders, often over a shirt, blouse, T-shirt, or other top, but sometimes next to the skin. Sweaters were traditionally made from wool but can now be made of cotton, synthetic fibers, or any combination of these.
According to British dictionaries, "sweater" is used in British English in the same sense as in American English but "jumper" is commonly used instead (though some say that "sweater" is used for heavier ones worn for warmth). Oxforddictionaries.com states that in British usage sweaters are always pulled over the head and jumpers are not necessarily, whereas most or all other British dictionaries disagree and say that sweaters are not necessarily pullovers or even say that jumpers are always pullovers, i.e. never open in front.
So according to most British dictionaries, British usage agrees with what American dictionaries describe as American English usage, according to which a sweater is either a pullover or a cardigan (which opens at the front). In other words, almost all British dictionaries include cardigans as a type of sweater but at least one includes cardigans as a type of jumper (i.e. most British dictionaries consider "sweater" – and at least one considers "jumper" – to be a hypernym for both pullovers and cardigans).
Source : Sweater Wikipedia