The usual name for the configuration file used by sendmail, usually residing in /etc.
Even modern versions, full of comments, cannot disguise the fact that the syntax resembles nothing so much as line noise, particularly when you get into the rulesets which actually define how addresses get manipulated into a form which it can understand. Sendmail's greatest blessing and greatest curse are one and the same: the fact that it began life in the days when the Internet was truly a network of networks, with each large member network having its own way of addressing email. Sendmail could, and did (and sometimes still does), understand email addresses like ...firstname.lastname@example.org. (And if anyone still has an email address like that, they need to get with it -- Internet access isn't expensive anymore...)
But for that power and flexibility, sendmail needed to be as general-purpose as possible -- leading to the monstrosity of syntax that is its config file. The most amusing example of sendmail.cf abuse is the infamous "Desk Calculator.sendmail.cf" file, which took the desired arithmetic operation as standard input and would send the correct answer to standard output.