- any element in any of the series of elements with atomic numbers 21–29, 39–47, 57–79, and 89–107, that in a given inner orbital has less than a full quota of electrons.
- any element belonging to one of three series of elements with atomic numbers between 21 and 30, 39 and 48, and 57 and 80. They have an incomplete penultimate electron shell and tend to exhibit more than one valency and to form complexes
