The phrase "middle initial" refers to the letter that is in the middle of a person's full name. It usually represents their second name or their middle name. Full definition
Putting your middle name or middle initial on your resume is fine. (uptowork.com)
Because of an editing error, an earlier version of a picture caption with this article gave an incorrect middle initial for the former mayor. (nytimes.com)
Also, in the author field, I put my first name and middle initial as asked, but I'd rather publish using my first initial and middle name. (lulu.com)