The term turtle applies to all forms of shelled reptile within the order Testudines.
However, confusion may arise from the words terrapin, tortoise, or seaturtle (also written as sea turtle). These terms apply to specialized forms of turtles. The word terrapin is usually reserved for freshwater or brackish dwelling turtles, while tortoise is used for primarily terrestrial species within the family Testudinidae. The term seaturtle is for marine forms. Certain types of pond and river turtles are also referred to as sliders or cooters, from the genera Trachemys and Pseudemy.
However, the ways in which these names are used can differ depending on what part of the world you are in, and the names may also be used interchangeably to describe different types of turtles.
Ultimately, they are all turtles, and the term turtle can be used to describe any of these reptiles regardless of where they are predominantly found.