A bullshit, pointless unit that you cover in geometry class that has nothing to do with math whatsoever. Created just to take up space within the school curriculum. There's also a converse, inverse, and contrapositive to a conditional, which just make it sound more and more stupider than it already is.
Conditional: f you are a musician, then you are a pianist.
CONVERSE: If you are a pianist, then you are a musician.
INVERSE: If you are not a musician, then you are not a pianist
CONTRAPOSITIVE: If you are not a pianist, then you are not a musician.