Identify the term and substance for the type of reaction Alkene undergoes

Part: 1: Addition reaction:

An addition reaction is an organic reaction in which two or more molecules combine to generate a bigger one.
Alkenes generally undergo addition reactions because alkenes have double bonds which are unstable and double bond breaks and provide a site for addition.

Part 2: Example showing addition reaction in alkene:
Example - Ethene $\left({\mathrm{C}}_2{\mathrm{H}}_4\right)$ is treated with $({\mathrm{Br}}_2)$ in the presence of Carbon tetrachloridechlo ride $\left({\mathrm{CCl}}_4\right)$ to produce an additional reaction product ${\mathrm{CH}}_2\mathrm{Br}-{\mathrm{CH}}_2\mathrm{Br}(1,2-\mathrm{dibromoethane})$

The reaction is as follows: $\underset{\mathrm{Ethene}}{{\mathrm{H}}_2\mathrm{C}={\mathrm{CH}}_2}\stackrel{{\mathrm{Br}}_2}{\left(\mathrm{g}\right)\to }\underset{1,2-\mathrm{dibromoethane}}{{\mathrm{CH}}_2\mathrm{Br}-{\mathrm{CH}}_2\mathrm{Br}\left(\mathrm{l}\right)}$

Thus, Alkene undergoes an addition reaction.