whom
pron.pron. the form of 'who' used as the object of a verb or after a preposition. You use it when the person you are talking about is receiving an action rather than doing it.
pron. the objective case of the relative or interrogative pronoun 'who'. Used as the object of a verb or following a preposition; increasingly restricted to formal registers in modern English.
To whom should I address this letter?
The manager, whom the staff respected deeply, decided to retire after thirty years of service.
In formal academic writing, one must distinguish between the subject who performs the action and the object whom the action affects, particularly when following a preposition.
From Middle English whom, wham, from Old English hwām, hwǣm, from Proto-Germanic hwammai, dative case of hwaz (“who, what”). Cognate with Scots wham (“whom”), German wem (“whom, to whom”), Danish hvem (“who, whom”), Swedish vem (“who, whom”).
Used as the object of a verb or preposition. In modern speech, 'who' often replaces it except when immediately following a preposition.
The person who I gave the book toThe person to whom I gave the bookWhile 'who' is common in speech, 'whom' is required in formal writing if it follows a preposition directly.