For a partial order, the size of the longest antichain is called the width.
Partial Order Width
See also
Antichain, Partial Order, Partial Order LengthExplore with Wolfram|Alpha
Cite this as:
Weisstein, Eric W. "Partial Order Width." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/PartialOrderWidth.html