initial side
noun
: a stationary straight line that contains a point about which another straight line is rotated to form an angle compare terminal side
Example Sentences
Recent Examples on the Web Second, although the initial side effects of the vaccine can be rough for people with long COVID symptoms, there are many case reports of the symptoms improving after vaccination. 
Dr. Keith Roach, oregonlive, 16 July 2021
Dr. Keith Roach, oregonlive, 16 July 2021 Word History
First Known Use
1957, in the meaning defined above