Halogen
Definition
The group 17 elements: fluorine, chlorine, bromine, iodine. They are aggressive nonmetals that desperately want one more electron, which makes them great disinfectants but also dangerous.
Technical definition
Halogens are the group 17 elements (F, Cl, Br, I, At) with seven valence electrons in an ns^2 np^5 configuration. They are highly electronegative nonmetals that readily gain one electron to form -1 ions, and their reactivity decreases down the group.
Example
Chlorine gas disinfects swimming pools, fluoride ions protect teeth, and iodine tincture disinfects wounds: same family, very different uses.
Common confusion
Halogens as elements are toxic and reactive, but their ions are often harmless and essential. Chloride in table salt is nothing like chlorine gas.