The phrase "subject line" refers to a short description or summary written in the email's or message's header. It gives the recipient a quick idea about the content or purpose of the message before opening it. Full definition
Please reply with a photo and write in subject line «no games» Thank you! (datingintimate.com)
If either the From or Subject line of an email raises any level of suspicion, it's probably spam. (lawyerist.com)
Don't pay attention to the television ads promising quick fixes or the many junk e-mails with subject lines claiming secret ways to improve your credit. (bostonglobe.com)