I think the fitting definition in its shortest form may be "a nanocrystal that displays quantum mechanical properties.

I would go with Toka's interpretation, but would add that the excitons are confined in all three spatial dimensions (quantum confinement)

A semiconductor or metal that shows quantum confinement effect due to reduction in grain size, is called quantum dot. In QD the quantum confinement direction/dimension is 3. In SCs QC occurs if the grain size is equal to or less than the exiton Bohr radius, while in metal, if the grain size is equal to or less than the mean free path of charge carrier.

A semiconductor or metal that shows quantum confinement effect due to reduction in grain size, is called quantum dot. In QD the quantum confinement direction/dimension is 3. In SCs QC occurs if the grain size is equal to or less than the exiton Bohr radius, while in metal, if the grain size is equal to or less than the mean free path of charge carrier.

Semiconductor nanocrystals that have tunable band gap by changing their particle size. moreover, excitons are confined in all three spatial dimensions. Their electronic properties are between those of bulk semiconductors and of discrete molecules.

every particle as 3 dimension (i.e. length, breath, height), when these 3 Dimension of a particle is confined to less than 1nm it is called Quantum dot.

Karimat El-Sayed, Physics department , faculty of science .Ain-Shams university, Egypt

the quantum Dots are Nano size grains which have sizes in the range from (1-10nm), each size will have its quantum-confined photonic and electronic properties.

nanosized object allowing the confinement of carriers in 3D

It is very good question and well stated by the people above....most of the terms used were attribute of quantum dots and some property was about dimension confinement.

"Apart from 3D confined structure the important thing that I found is important for Quantum dots is it should have the radius very close or lesser than Bohr's radius value" it is the position where particle shows its specific quantum mechanical property as stated by Toka"

I think the Wikipedia article on quantum dots and the definition given there are quite good. If any of you disagree, it is easy to offer your corrections to Wikpedia. See:

https://en.wikipedia.org/wiki/Quantum_dot

and for info on editing Wikipededia articles, see

https://en.wikipedia.org/wiki/Wikipedia:Editing_policy

It may be the exact definition

Any nanocrystalline material which confines its excitons in all three spatial directions can be defined as Quantum dots.

https://spie.org/samples/PM129.pdf

Quantum dots are semiconducting florescent nanocrystals where the electrons and holes are confined in all three spatial directions. They are made of few numbers of atoms (200-1000) with an inorganic layer (surface ligand). QDs possesses size-dependent optical, magnetic and electric properties when the size varies below its bulk exciton Bohr radius.