vicis

This word has no nominative singular. The dictionaries head it with the genitive vicis for want of anything better, and what actually exists is vicem, vice, and the plural vices, vicium, vicibus. A defective noun of exactly this shape is rare, so the gap is worth remembering rather than hunting for.

The core idea is one thing taking another's place. Vice plus a genitive means "instead of", which is where English vice-president and viceroy come from, and the Latin phrase vice versa, "the position having been turned", is still doing service unchanged. In vicem and invicem mean "in turn, mutually", and vices in the plural are the alternations of fortune, the seasons or a rota of duty. Do not confuse any of it with vicus, "village, street", a separate second declension noun.