In dehydrohalogenation, hydrogen and halogen are removed from

Dehydrohalogenation means removal of hydrogen and halogen from a molecule. Usually bases can be used for the dehydrohalogenation reaction to form double bond between two carbon atoms. For the formation of double bond removal of hydrogen and halogen must be occur on adjacent carbon atoms. 

${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2\mathrm{Cl}+\mathrm{base}\to {\mathrm{CH}}_3\mathrm{CH}={\mathrm{CH}}_2$