When the sets A and B are two events associated with a sample space. Then, event $\text{ ' }\mathrm{A}\cup \mathrm{B}\text{ ' }$denotes 

Recall that union of two sets A and B denoted by $A\cup B$ contains all those elements which are either in A or in B or in both. When the sets A and B are two events associated with a sample space, then $A^{\mathrm{\prime }}\cup B^{\mathrm{\prime }}$ is the event 'either A or B' or both'. This event ${\mathrm{A}}^{\mathrm{\prime }}\cup {\mathrm{B}}^{\mathrm{\prime }}$is also called $\text{ 'A or B' }$.

Therefore, event ${ }^{\text{'}}A\text{ or }{B}^{\mathrm{\prime }}=A\cup B$

$=\{x:x\in \mathrm{A}\text{ or x}\in \mathrm{B}\}$