Beachhead is a military term used to describe the line created
when a unit (by sea) reaches a beach, and begins to defend that
area of beach, while other reinforcements (it is hoped) help
out, until a unit large enough to begin advancing has arrived.
Sometimes used interchangeably (both correctly and incorrectly)
with Bridgehead and Lodgement. Beachheads were very important in
operations such as Operation Neptune, the Korean War (especially
at Inchon), and the Vietnam War, among many other examples.
Although many references state that Operation Neptune refers to
the naval operations in support of Operation Overlord, the most
reliable references make it clear that Overlord refers to the
establishment of a large-scale lodgement in Normandy, and that
Neptune refers to the landing phase which created the beachhead;
i.e. Neptune was the first part of Overlord. According to the
D-day museum:  "The armed forces use codenames to refer to the
planning and execution of specific military operations.
Operation Overlord