whom
Whom is used in formal or written English instead of `who' when it is the object of a verb or preposition.
1 quest You use whom in questions when you ask about the name or identity of a person or group of people.
(=who)
`I want to send a telegram.'<emdash10001`Fine, to whom?'..., Whom did he expect to answer his phone?...
2 conj You use whom after certain words, especially verbs and adjectives, to introduce a clause where you talk about the name or identity of a person or a group of people.
(=who)
He asked whom I'd told about his having been away...
3 pron You use whom at the beginning of a relative clause when specifying the person or group of people you are talking about or when giving more information about them.
oft prep PRON
One writer in whom I had taken an interest was Immanuel Velikovsky...
Translation English - Cobuild Collins Dictionary  

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"Collins Cobuild English Dictionary for Advanced Learners 4th edition published in 2003 © HarperCollins Publishers 1987, 1995, 2001, 2003 and Collins A-Z Thesaurus 1st edition first published in 1995 © HarperCollins Publishers 1995"