invicem

Invicem is a frozen phrase, in plus the accusative of vicis, "turn, change", written as one word. Classical Latin uses it for alternation: two things taking it in turns, or one thing swapping with another. Cicero would say invicem and mean "by turns".

The church corpus stretched it into something Latin badly needed. Latin has no reciprocal pronoun, no single word for "each other", so translators of the Greek New Testament pressed invicem into that job: diligite invicem, "love one another", portate alterutrum onera. That reciprocal sense is now the commonest one in ecclesiastical and medieval texts, and it is where the awkward English "in turn" often will not fit. Manuscripts spell it inuicem, which is the same word with the old u for v.