fidei

Fidei is a Latin term meaning “of faith” or “of trust.” It is the genitive singular of fides, a feminine noun whose core senses include faith, trust, reliability, and a pledge. The word fides derives from the verb fido, “to trust,” and in Latin it spans both secular and religious contexts, from personal fidelity to the theological virtue of faith.

As a genitive form, fidei is used to express possession or attribution in Latin phrases and inscriptions. It frequently appears in ecclesiastical and historical contexts, signaling relationships such as “the faith of X” or “the trust of Y.” A well-known modern usage is the title Fidei Defensor, or Defender of the Faith, a Latin motto associated with the English crown, in which fidei means “of the faith.”

Beyond titles, fidei appears in various Latin texts within phrases related to faith, trust, or obligation, and in translations of Christian doctrinal statements where fides (faith) is qualified by the genitive to indicate what the faith pertains to.

In contemporary scholarship, fidei is treated as a standard genitive form of a core noun and is encountered across grammars, lexica, and translations of Latin sources. Related terms include fides (faith), fiducia (trust), and fideismus (fideism).