An orbital is best described as the electron shell closest to the nucleus. the outermost electron shell of an atom. the volume of space in which electrons are most often found. the original energy level of electrons in photosynthesis.

           An Orbital is best described as the volume of space in which electrons are most often found.

Explanation:

                  As we know atom consists of sub-particles commonly known as protons, neutrons and electrons. The outer space around the nucleus where the probability of finding electrons is maximum is known as orbital. As the electrons are not precisely ordered around the nucleus hence it is not easy to tell the exact position of an electron.

                 Hence, four quantum numbers are used to locate the position of electrons around the nucleus.

   i) Principle Quantum Number:

                                                    This number explains the main energy level which tend to increase in energy as the distance of electrons from nucleus are increased. Principle Quantum Numbers are integer number ranging from one to infinity. Hence, increase in this quantum number results in increase of the size of orbital.

   ii) Azimuthal Quantum Number:

                                                      This Quantum Number explains the direction of particular orbital in 3-dimensional space. Also it is responsible for the shape of an orbital.

  iii) Magnetic Quantum Number:

                                                       This Quantum Number also tells the direction of orbital in 3D space with respect to x, y and z axis.

  iv) Spin Quantum Number:

                                               This Quantum Number tells about the spin direction of an electron about its axis which may be clockwise or anticlockwise.