sinus node
noun
: a small mass of tissue that is embedded in the musculature of the right atrium of higher vertebrates and that originates the impulses stimulating the heartbeat
Example Sentences
Recent Examples on the Web Hearts beat when the sinus node, a small mass of tissue in the right upper chamber, transmits an impulse to the heart cells. 
Michael Ciaglo, National Geographic, 5 June 2020 Over the course of a day, a person’s heart beats about 100,000 times, and each beat begins with an electrical signal generated in the sinus node in the heart’s upper right chamber. Emily Sohn, Washington Post, 7 Mar. 2018
Michael Ciaglo, National Geographic, 5 June 2020 Over the course of a day, a person’s heart beats about 100,000 times, and each beat begins with an electrical signal generated in the sinus node in the heart’s upper right chamber. Emily Sohn, Washington Post, 7 Mar. 2018 Word History
First Known Use
1937, in the meaning defined above
Medical Definition
sinus node
noun
: sinoatrial node