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:
Once an amphibious assault starts, victory tends to go to the side which can reinforce the beachhead most quickly. There are exceptions to this rule where the amphibious forces have not expanded from their beachheads quickly enough to create a lodgement area before the defenders can reinforce their positions. Two famous examples in which the attackers failed to expand their beachheads before the defending side could bring up reinforcements occurred during the landing at Suvla Bay in the Gallipoli Campaign in World War I, and the amphibious landing at Anzio during World War II.