Hold one picture and every use of this word falls into place: two things born of the same parent. Latin cognatus means born together, and that is exactly what the word still does — it puts two items on one family tree and says they came down from the same source. Change what sits on the tree and you get each of the senses: put people on it and you get blood relatives; put words on it and you get English house beside German Haus; put subjects on it and you get history beside archaeology. The word never measures how alike two things look now. It only says where they came from.
Borrowed from Latin cognatus, related by blood, from co- (together) plus gnatus (born). A doublet of connate.
借自拉丁文 cognātus（有血緣的），由 co-（一起）加 gnātus（出生的）組成，字面就是「一起出生」。英文 connate 是它的同源雙生字。
Wiktionary entry for cognate supplied in this packet (source_ref kaikki:3f6f69a190036ae4:496688944)
Three branches
語言上的同源
把家族樹上的點換成「字」和「語言」：兩個字如果追得到同一個老字，就像同一個親代生出來的兩個孩子。這是現代英文裡最常見的一層，也是學語言的人唯一會主動用到的一層。
adjective · C1
Cognate words or languages grew out of the same older word or the same older language, so they are related in the way two members of one family are.
同源的
血緣上的同源
這是最貼著拉丁文原意的一層，樹上放的就是人。它沒有比喻，字面就是「一起出生」。也因為這樣，它反而變成最老、最侷限的一層：今天只活在法律文件和家族史裡。
adjective · C2
Related by birth; belonging to the same family by blood. In legal use it points in particular at relatives on the mother's side.
血親的（法律上多指母系）
廣義的同一類
樹上放的不再是人也不是字，而是學科、想法、說法。血緣的畫面被整個借走：兩個領域如果從同一個根長出來、還共用工具和問題，就說它們 cognate。
adjective · C2
Belonging to the same group, or coming from the same source, and so close to each other in nature. Used in formal writing about subjects, fields of study, ideas or statements.
同一類的、同一脈絡的