invicem

Adverb

 * 1) in or by turns; alternately
 * 2)  mutually, each other, reciprocally, one another
 * 1)  mutually, each other, reciprocally, one another

Usage notes
This word is attested in Plautus and Golden Latin authors (e.g. Livy) in the sense of 'each other, mutually', but very rarely. This usage becomes common in Silver Latin and afterwards.

Etymology 2
From a reinterpretation of the adverb invicem as an accusative-case form.

Pronoun

 * 1)  each other