What is electron cloud $?$

Hint: Electron cloud is related to the atom, in which the cloud-like formation of electrons takes place around a nucleus of atom. There are different regions of densities of electrons. Some are high density electron regions while some are low density electron regions.

Complete answer:
When the electron was discovered in $1897$ by J.J Thomson, there were many theories proposed by various scientists about the motion of these electrons in the atom. Bohr proposed the theory that these electrons revolve around the nucleus in their particular shell. He compared the revolution of electrons to the revolution of planets around the sun in a fixed orbit. Another famous theory came up, according to which electrons do not revolve in any orbit. But in actuality they are in cloud formation around the nucleus of the atom. This refers to the electron cloud. Thus we may define electron clouds as the area around the nucleus of an atom where the probability of finding electrons is maximum. Therefore the electron cloud is a region of a group of electrons around a nucleus of the atom. This electron cloud is darkest at the centre of the atom and goes lighter as we move away from the centre of the atom. This depicts that electrons are most likely to be present near the centre of the atom which is the nucleus region.

Note:
Electron cloud is a pictorial representation of the density of electrons inside the atom. The darkest region is the most densely electronic region and lighter region indicates the low probability of finding an electron. Thus there is no representation of shells in electron clouds.