What is a terminal atom and why are hydrogen and the halogen elements considered terminal atoms?

Terminal atoms -

The atoms that are always present at the terminal position, because they have the capability of bonding via one single bond.

hence,

Hydrogen atom is a terminal atom, because it can only form one single bond, since, hydrogen has only one electron in its shell, so the octet is completed by sharing that electron with another one electron, and forming a bond.

Therefore, hydrogen atom is considered to be a terminal atom.

Similarly, Halogens have 7 electrons, hence, to attain noble gas configuration, it require only one electron. Hence, it can form only one single bond.

Therefore, halogen atom is considered to be a terminal atom.