An event which has only sample point of a sample space, is called simple event. 

If an event E has only one sample point of a sample space, then it is called a simple ( or elementary) event. In a sample space containing n distinct elements, there are exactly n simple events. 

For example, in the experiment of tossing two coins, a sample space is

$\mathrm{S}=\{\mathrm{HH},\mathrm{HT},\mathrm{TH},\mathrm{TT}\}$

There are four simple event corresponding to this sample space. There are ${\mathrm{E}}_1=\left\{\mathrm{HH}\right\},{\mathrm{E}}_2=\left\{\mathrm{HT}\right\}$, ${\mathrm{E}}_3=\left\{\mathrm{TH}\right\}$ and ${\mathrm{E}}_4=\left\{\mathrm{TT}\right\}$