A container is an object holding a collection of elements, so we call Vector a container because it is the type of objects that are containers. [0]

Containers are used to manage objects of a given type. The STL containers are divided into three categories: Sequence containers (also called sequential containers) Associative containers Container adapters [1]

[0] The C++ Programming Language 4th ed.

[1] C++ Programming From Problem Analysis to Program Design