What is the Avatar according to common hindus?

A comman hindu believe that God Almighty comes down to the earth in some bodily form to protect the religion. to set an example or to set the rules for humanbeings.There is no reference of Avatars anywhere in the vedas,the most sacred of the hindu scriptures i.e.saruti.However it is found in the smrthi i.e.the puranas and the Ithihasas.