Main Difference
The main difference between Shirt and Jacket is that the Shirt is a garment for the upper body and Jacket is a clothing for the upper body.
Shirt
A shirt is a cloth garment for the upper body (from the neck to the waist).
Originally an undergarment worn exclusively by men, it has become, in American English, a catch-all term for a broad variety of upper-body garments and undergarments. In British English, a shirt is more specifically a garment with a collar, sleeves with cuffs, and a full vertical opening with buttons or snaps (North Americans would call that a “dress shirt”, a specific type of “collared shirt”). A shirt can also be worn with a necktie under the shirt collar.
Jacket
A jacket is a mid-stomach–length garment for the upper body. A jacket typically has sleeves, and fastens in the front or slightly on the side. A jacket is generally lighter, tighter-fitting, and less insulating than a coat, which is outerwear. Some jackets are fashionable, while others serve as protective clothing.
Shirt (noun)
An worn on the upper part of the body, and often has sleeves, either long or short, that cover the arms.
Shirt (noun)
a member of the shirt-wearing team.
Shirt (verb)
To cover or clothe with a shirt, or as if with a shirt.
Jacket (noun)
A piece of clothing worn on the upper body outside a shirt or blouse, often waist length to thigh length.
Jacket (noun)
A piece of a person’s suit, beside trousers and, sometimes, waistcoat; coat US
Jacket (noun)
A removable or replaceable protective or insulating cover for an object (e.g. a book, hot water tank.)
Jacket (noun)
A police record.
Jacket (noun)
In ordnance, a strengthening band surrounding and reinforcing the tube in which the charge is fired.
Jacket (noun)
The tough outer skin of a baked potato.
“Cook the potatoes in their jackets.”
Jacket (verb)
To enclose or encase in a jacket or other covering.
Shirt (noun)
a garment for the upper body made of cotton or a similar fabric, with a collar and sleeves, and with buttons down the front
“tonight he’s smartly dressed in shirt and tie”
Shirt (noun)
a garment similar to a shirt, made of stretchable material and typically having a short row of buttons at the neck, worn as casual wear or for sports
“a rugby shirt”
Shirt (noun)
used to refer to membership of a particular sports team
“Smith increased his chances of a Great Britain shirt with a penalty shot save”