A computer that harnesses phenomena from quantum mechanics in order to perform much more efficient computations than older, classical computer technologies are capable of.
Superposition and entanglement are the two features of quantum mechanics used for quantum computations. These features empower quantum computers to handle operations at speeds that are exponentially higher as compared to traditional computers, while also consuming a lot less energy in the process.
A very simple definition of a quantum computer is a computer that harnesses phenomena from quantum mechanics in order to perform much more efficient computations than older, classical computer technologies are capable of.
The field of quantum computing originates in the 1980s, when it was discovered that certain computational problems can better be tackled using quantum algorithms than with the classical counterparts in use at the time.
Quantum computing could significantly contribute to the fields of finance, military affairs, artificial intelligence, big data, etc.