Real Dictionary

blackmail
uncountable noun: Blackmail is the action of threatening to reveal a secret about someone, unless they do something you tell them to do, such as giving you money.
uncountable noun: If you describe an action as emotional or moral blackmail, you disapprove of it because someone is using a person's emotions or moral values to persuade them to do something against their will.

blackmail in American English
a tribute paid to freebooters and bandits along the Scottish border to assure safety from looting
to get or try to get blackmail from
noun: any payment extorted by intimidation, as by threats of injurious revelations or accusations
transitive verb: to extort money from (a person) by the use of threats

blackmail in British English
to exact or attempt to exact (money or anything of value) from (a person) by threats or intimidation; extort
noun: the act of attempting to obtain money by intimidation, as by threats to disclose discreditable information