Abbreviation for Major Character Death, the phrase usually used by fans and in Fanfiction. An Archive Of Our Own warning.
Person 1: I wanted to read some fluffy Fanfiction last night, but I missed the MCD warning in the description! And now my heart is in pieces!
Person 2: MCD? Is that some form of OCD or something?
Person 1: *whimpers* It means Major Character Death.