The Collaborative International Dictionary of English v.0.48:
Achromatin \A*chro"ma*tin\, n. (Biol.) Tissue which is not stained by fluid dyes. --W. Flemming. [1913 Webster]WordNet (r) 3.0 (2006):
achromatin n 1: the part of a cell nucleus that is relatively uncolored by stains or dyes