In physics, the adjective semiclassical has different precise meanings depending on the context. Physics (Greek Physis - φύσις in everyday terms is the Science of Matter and its motion. All these meanings usually refer to some approximation, limit or situation that combines quantum and classical aspects in a given problem. Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons Classical mechanics is used for describing the motion of Macroscopic objects from Projectiles to parts of Machinery, as well as Astronomical objects Some of the possible significations are the following:

First, semiclassical approximation may refer to quantum-mechanical calculations that are obtained by considering a small perturbation of a classical calculation, for example the WKB approximation in non-relativistic quantum mechanics or the loop expansion or the instanton methods in quantum field theory. In Physics, the WKB (Wentzel–Kramers–Brillouin approximation also known as WKBJ (Wentzel–Kramers–Brillouin–Jeffreys approximation is the most familiar Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons An instanton or pseudoparticle is a notion appearing in theoretical and Mathematical physics. In quantum field theory (QFT the forces between particles are mediated by other particles In quantum field theory, a semiclassical correction arises from one-loop Feynman diagrams. In Physics, a one-loop Sander-Feynman diagram is a connected Feynman diagram with only one cycle ( Unicyclic) The semiclassical effective action is

$\Gamma[\phi]=S[\phi]+\frac{1}{2}\textrm{Tr}\left[\ln {S^{(2)}[\phi]}\right]+...$

Second, in the context of open quantum systems and measurement theory, where one considers the dynamics of a given quantum system in interaction with an environment, the semiclassical regime may refer to the situation in which the wavefunction of the system is approximately peaked around the solution of the corresponding classical equations of motion. In Quantum field theory, the effective action is a modified expression for the action, which takes into account Quantum-mechanical corrections in the following In Physics, an open quantum system is a quantum system which is found to be in interaction with an external quantum system the environment. Corrections to the classical trajectory and the dispersion of the solution around the mean value are usually considered.

Third, semiclassical gravity is the approximation to the yet unknown theory of quantum gravity in which one treats matter fields as being quantum and the gravitational field as being classical. Semiclassical gravity is the approximation to the theory of Quantum gravity in which one treats Matter fields as being quantum and the gravitational field Quantum gravity is the field of Theoretical physics attempting to unify Quantum mechanics, which describes three of the fundamental forces of nature The classical Einstein equations are computed with the expectation value of the quantum matter fields in the classical background. Semiclassical gravity has applications in black hole physics and physical cosmology. A black hole is a theoretical region of space in which the Gravitational field is so powerful that nothing not even Electromagnetic radiation (e Physical cosmology, as a branch of Astronomy, is the study of the large-scale structure of the Universe and is concerned with fundamental questions about its

A semiclassical approximation is any high frequency approximation (or "high energy approximation"), less extreme than classical mechanics, that is used to approximate quantum mechanics. A high frequency approximation (or "high energy approximation" for Scattering or other Wave propagation problems in Physics or Engineering

Fourthly, certain early theories in quantum mechanics, notably the Bohr model of the hydrogen atom, are sometimes called semi-classical, as they incorporate the newer quantum ideas into an essentially classical framework. In Atomic physics, the Bohr model created by Niels Bohr depicts the Atom as a small positively charged nucleus surrounded by Electrons