Proof by Contradiction/Also known as

Proof Rule
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.

Some sources do not explicitly distinguish between Proof by Contradiction and Reductio ad Absurdum.

Both can be referred to as indirect proof.