I. əˈ- adverb
Etymology: Middle English abord, from a- (I) + bord board, side of a ship — more at board
1.
a. : on board : on, onto, or within a ship, a railway car, or a passenger vehicle
all aboard
climb aboard
b. : astride
swing aboard
sling a saddle aboard
2. : alongside
another ship close aboard
II. preposition
Etymology: Middle English abord, from abord, adverb
: on board
go aboard ship
aboard a horse
III. adverb (or adjective)
1. : in or into a group, association, or organization
her second promotion since coming aboard
2. baseball : on base