The general understanding are that oceans, while they comprise the largest bodies of water on Earth, and that they are the largest part of Earth, are defined by a variety of standards, while seas are defined by an equally differing variety of standards.
The general definition of ocean is that it is a major body of salty water that is a continuous body of water that covers about 70 percent of the Earth’s surface. This continuous body of water has divisions into smaller oceans and even the seas.
In other words, the world has one global, connected body of salt water that is called the World Ocean or global ocean. This is an important concept because of the mechanisms of the atmosphere and water that interchange freely among the parts of the world ocean.
There are criteria for defining the divisions of the world ocean. There are continental divisions, archipelagos, the equator, the major latitude divisions, the hemispheres, and so on.
The Caspian and Black Seas are also considered as seas that are separate from the globally interconnected World Ocean, making them two of the three World Oceans in some classifications, and making them seas in other classifications.
In terms of continental separation: The Pacific Ocean separates Asia from the Americas. The Atlantic Ocean separates the Americas from Eurasia and Africa, and so on. There are also the Indian, Southern and Arctic Oceans of the major oceans of the Earth.
Then the Pacific and the Atlantic are divided by the equator into the North Pacific and Atlantic and the Southern Pacific and Atlantic. After this major type of hemispheric division, there are the seas, gulfs, straits, bays and other titles that are based on the features of the land as it interacts with the ocean.
Seas are saline bodies of water that are either completely enclosed by land, such as the Salton Sea, or the Aral Sea. These are also referred to as salt lakes and not technically seas that are part of the World Ocean.
Seas can be enclosed on three sides, such as the Sea Of Cortez. Again, the Black and Caspian Seas are named as seas but are also considered to be oceans.
As a result, it’s all in a name, the interconnectivity, the separation of continents, whether landlocked or not, and whether of great size as an ocean, or smaller size as a salt lake that is called a sea!