Proof by Contradiction/Also known as
Proof by Contradiction is also known as not-introduction, and can be seen abbreviated as $\neg \mathcal I$ or $\neg_i$.
However, there are technical reasons why this form of abbreviation are suboptimal on this website, and PBC (if abbreviation is needed at all) is to be preferred.
It is also known as proof of negation, i.e., proof that some (positive!) assumption is not true.