Real Dictionary

nightrider in American English
any of a band of masked, mounted men who perform lawless acts of violence and terror at night to intimidate, terrorize, etc.; esp., any of such a band of white men in the S U.S. after the Civil War
noun: one of a band of mounted men, esp. in the southern U.S. during Reconstruction, who committed nocturnal acts of violence and intimidation against African-Americans and people who were sympathetic to them

nightrider in British English
noun: a member of a band of mounted and usually masked White people in the southern US who carried out acts of revenge and intimidation at night after the Civil War