What‌ ‌are‌ ‌stereoisomers?‌ ‌Give‌ ‌an‌ ‌example‌

Hint: Isomers are molecules which have the same number of atoms that are the same empirical formula but vary in the way the atoms are arranged.

Complete answer:
Stereoisomers are molecules with the same molecular formula that vary only in how their atoms are structured in three-dimensional space, and there are multiple subcategories under the stereoisomer group. Geometrical isomers and optical isomers are the two major groups of stereoisomers.
-Geometric isomers: Geometric isomers, also known as cis or trans isomers, are stereoisomer types that arise from a double bond or a ring structure.
Examples of geometric isomers are $cis - 2 - butene$ and $trans - 2 - butene$.
The double bond in each molecule is between carbons $2$ and $3$. The methyl groups bound to carbons $2$ and $3$ are on the same side of the rigid double bond in $cis - 2 - butene$. The methyl groups of $trans - 2 - butene$ are on opposite sides of the double bond.
-Optical isomers: Optical isomers are molecules with mirror images of their shapes that cannot be superimposed in any direction. These molecules are known as chiral because they lack the plane of symmetry. Enantiomers are the d and l isomers of a compound. In octahedral complexes with $1,2,$ or $3$ symmetrical bidentate ligands, optical isomerism is normal. Chiral compounds are molecules that are not superimposable mirror images of each other, and this property is known as chirality. A plane of symmetry does not exist in optical isomers.
Example of Optical isomer is ${\left[ {Co{{\left( {Cl} \right)}_2}{{\left( {en} \right)}_2}} \right]^ + }$

Note:
Stereoisomers that do not mirror images of one another and cannot be superimposed over one another are called diastereomers. Diastereomers are stereoisomers which have two or more stereocenters. It can be difficult to tell whether two molecules are diastereomers or not.