By-phrase

In syntax, by-phrase is an optional adjunct in a passive construction headed by by and containing the logical subject.

Examples
In (i) the by-phrase contains Vitesse, the logical subject of won (cf. Vitesse won the match). As shown by (ii), by-phrases are also possible in nominal passives.

(i)  The match was won by Vitesse.

(ii) Carthago's destruction by the Romans.

(iii) They kissed by the light of the moon.

Not all languages allow passive by-phrases, and not all phrases headed by by are passive by-phrases: in (iii) the light of the moon is not interpreted as an argument of the verb kiss.

Link
Utrecht Lexicon of Linguistics