We have had several threads on how a particular word gets into a dictionary, but now I'm wondering the opposite. I am re-reading a Mary Stewart novel, vintage 1967, in which the main character and her companion come across a Chambers Dictionary, from which the main character reads the word cusk, and asks whether her companion knew that a cusk is a torsk or a burbot. Well, MY newish Chambers being to hand (hi, Jo!), I found that it contained torsk and burbot but not cusk. So--how is it decided that a word should be dropped from a dictionary?

Dear Jackie: Chambers may not list "cusk" but my dictionary does.
cusk
n.,
pl. cusk or cusks 5Brit local name < ?6
1 a large, edible gadoid fish (Brosme brosme) found in the N Atlantic
>2 BURBOT

I just checked, and AHD has it also

but to hurriedly back away from the comparison of dictionaries and attempt to answer the question, it's all a matter of policies and standards. the editorial board (or perhaps the usage panel) will decide how to apply markings such as obsolete and archaic and rare and under what circumstances a word so classified will be dropped altogether.

a diachronic, or historical, dictionary such as OED will not soon drop any word; a synchonic dictionary such as W3 will be under constant revision in this regard.

Wow--hooray for Atomica! Diachronic and synchronic are exactly what I thought they were, going by your context, tsuwm. Thanx for 2 new words! (And for the other info. I just thought it was interesting that though we have several times brought up how words get into a dictionary, it had not occurred to me that they are dropped.)