Technically it's a flag staff. The jack staff is for a second flag, or jack, and is mounted at the bow.
[quote]10. (Naut.)
(a) A flag, containing only the union, without the fly,
usually hoisted on a jack staff at the bowsprit cap;
-- called also union jack. The American jack is a
small blue flag, with a star for each State.
[1913 Webster]
http://onlinedictionary.datasegment.com/word/jack+staff
http://en.wikipedia.org/wiki/Maritime_flag#Jacks
http://en.wikipedia.org/wiki/Jackstaff