(covers information from several alternate timelines)
The Atlantic Ocean was an Earth ocean. The equator, a line of latitude which passed through this ocean, divided it into two parts, the North Atlantic Ocean and the South Atlantic Ocean. Four continents - Europe, Africa, South America, North America - had coastlines on this ocean, the latter's coastline known as the East Coast. An underwater mountain range, the Mid-Atlantic Ridge, bifurcated the ocean floor. (TOS-R: "The Cage"; TNG: "Family")
In an alternate 1944, after conquering England, the forces of Nazi Germany crossed the Atlantic and invaded the northeastern United States. The ocean itself was labeled on the map of the United States of America that showed the progress of the Nazi's campaign on the North American continent. It was described there as Atlantischen Meer (Atlantic Sea). (ENT: "Storm Front", "Storm Front, Part II")
In the late 20th and early 21st centuries, the cruise liner RMS Queen Elizabeth 2 ferried passengers across the Atlantic between the continents of Europe and North America. One of these passengers was Ralph Offenhouse. (TNG: "The Neutral Zone")
A 21st century-era political map of Earth was stored in the USS Enterprise's library computer in 2254. The Atlantic Ocean was seen on this map. This map was among the materials viewed by the Talosians when they scanned the Enterprise computer. (TOS-R: "The Cage")
For the remastered "The Cage", a Central Intelligence Agency map was used, It was published either in April 2007 or September 2008. The difference, which was not noticeable at the resolution of the image from the episode, was the addition of Kosovo, which gained its independence between the releases. The labels "North Atlantic Ocean" and "South Atlantic Ocean" were illegible to the viewer.
Several 20th century-era maps and diagrams depicting the Atlantic Ocean, or portions of it, were stored in the USS Enterprise library computer in 2254. These included a diagram of the polar orbit of Nimbus 1, a continent projections map, an eastern hemisphere map, a political map of the northern hemisphere, and an orbital map. These maps were among the materials viewed by the Talosians when they scanned the Enterprise computer. (TOS: "The Cage")