What is the word for the hole or cup at the top of a knight's helmet from which a plume streams?
"Crest holder" or "crest support". See this: http://www.museumoflondonprints.com/image/78976/unknown-roman-helmet-crest-holder
Crest block. Go here: Roman helmet
For completeness, although this may not be the type of helmet originally in mind when the question was asked, Officers of the Household Cavalry in the UK wear a helmet with a plume similar to this:
[Life Guards helmet and plume from householdcavalry.info]
The mount which supports the plume is called a plume spike.