Let pn denote the n-th prime. The bounded prime gap constant is
C88a=H1:=n→∞liminf(pn+1−pn),
the least limit point of the sequence of gaps between consecutive primes. That H1 is finite — that some bounded gap recurs infinitely often — was proved by Zhang [Zha14] in 2013; the twin prime conjecture is exactly the assertion H1=2.
More generally one writes Hm:=liminfn→∞(pn+m−pn) for m≥1. Maynard [May15] proved Hm finite for every m; the growth rate of Hm in m is recorded separately as C88b.