Wiktionary
n. A hip-hop performer responsible for backup rapping and singing and increasing an audience’s excitement with call-and-response chants.
Wikipedia
A hype man in hip hop music and rapping is a backup rapper and/or singer who supports the primary rappers with exclamations and interjections, and who attempts to increase the audience's excitement with call-and-response chants.
Music writer Mickey Hess expands the term as follows: "a hype man is a figure who plays a central but supporting role within a group, making his own interventions, generally aimed at hyping up the crowd while also drawing attention to the words of the MC".
Discussing the role of the hype man in the book How to Rap, Royce da 5'9" describes how a hype man can contribute to a live performance: "a lot of my verses [can] be so constant with the flow [that] I'd need somebody to help me." Lateef has stated, "You're gonna have to have somebody say something somewhere to give you a breath... usually it's just a matter of getting somebody to hit some line or some word in a line - that's all you really need."