frater

Frater, fratris is third declension masculine, and it belongs to a small family group that all shorten the -er of the nominative before the case endings: pater and patris, mater and matris, frater and fratris. Learn the three together and none of them will surprise you.

In classical Latin frater is a brother by blood, and a cousin is frater patruelis, "father's-brother's brother". Roman family vocabulary is more precise than ours about which side of the family a relative sits on, and this is the mild end of it.

The word ranks higher here than Caesar would explain. In monastic and ecclesiastical Latin fratres are the brethren of a community, the standard way of addressing a congregation, and the title of the friars (French frere, English friar, both from this word). A corpus that mixes the classical authors with the Church lifts frater several dozen places. It is a straight Indo-European cognate of English brother rather than a loan, and it gives fraternal, fraternity and fratricide.