The name for all of Earth’s interconnected ocean waters as one system is the World Ocean.
Although maps divide the seas into the Pacific, Atlantic, Indian, Southern, and Arctic oceans, these divisions are geographic and political conventions rather than separate sealed bodies of water. Water can circulate between them through open ocean routes and connecting straits, so oceanographers often describe them collectively as one global ocean.
The World Ocean concept helps explain large-scale processes such as thermohaline circulation, in which differences in temperature and salinity drive deep and surface currents around the planet. It also emphasizes that pollution, warming, sea-level rise, and changes in marine ecosystems can cross regional boundaries.
“Global Ocean” is also a defensible synonym, but “World Ocean” is the standard canonical term used in this question. The term does not mean that every part of the ocean has identical conditions: temperature, salinity, depth, currents, and ecosystems vary dramatically from place to place.