flos

Flos, floris is third declension and masculine, which catches people out: nothing about a flower is grammatically feminine in Latin. The stem is flor-, so floris, florem, flore, flores, florum, floribus, and from it come floreo, "I bloom", flora, floridus and the goddess Flora.

The figurative sense is at least as common as the literal one. In flore aetatis is the prime of life, flos iuventutis is the pick of the young men, and by extension flos is the best part of anything, including the finest flour of the wheat. That last use is why medieval recipes and English "flower of wheat" eventually gave us the spelling flour. French fleur, Spanish flor and Italian fiore all descend from it, along with English flourish, floral and florist.