diamond principle

Also known as ◊

combinatorial principle that there exists a family of sets 𝐴(𝛼)⊆𝛼 for 𝛼<ω₁ such that for any 𝐴⊆ω₁, the set of 𝛼’s with 𝐴∩𝛼=𝐴(𝛼) is stationary in ω₁