trying to figure out a word

i remember reading about this several years ago and a recent article i read made me remember it but i cannot for the life of me remember what it was called or how it worked.

basically it's a method of determining on average how many bugs a certain ammount of lines contain.

Every program contains at least one bug, and can be shortened by at least one instruction. By induction, every program can be reduced to a single instruction that does not work.