What does dependency/dependant mean?

Terminology is context dependent. In general, the dependency is a synonym of dependence and describes not one entity, but state. According to Cambridge dictionary:

the situation in which you need something or someone all the time, especially in order to continue existing or operating.

So it's more about a relation to something. For example:

A minor child is dependent of his/her parents. The child is in a state of dependency. The dependency describes the relation between the child and its parents.

The google definition is only one of the available definitions (see for example the Collins dictionary), the one that is especially applicable in the context of geopolitics and real estate. For example:

Before 1776, America was a dependency of the British Empire. The dependency America was dependent on the Empire.

He bought a farm with several dependencies

The object oriented terminology is not so different from the more general definition: a dependency is a relationship between objects or classes. Here a more formal definition from the UML 2.5 standard:

7.7.1: A Dependency signifies a supplier/client relationship between model elements where the modification of a supplier may impact the client model elements.

So, restating what's in the tutorial, we can say that:

If A has to use B, A depends on B. There is a relation of dependency from A to B. A is said to be the dependent.

For practical reason and language convenience, B is often called the dependency (which is indeed the contrary of everyday's language). But in reality, it is still the relation of A to B that is the dependency.

A depends on B, is A dependant, or is B dependant?

A is dependent, it depends on B. If these were variables, B would be called an independent variable.

In programming, if A is a dependant of B, then B is a dependancy. This term is also fairly common in project and resource management.

However, in human relationships - if A is a child or spouse配偶 that is dependent受赡养者 on B, then A would remain a dependant but B would commonly be called a provider or maybe a supporter.