泛型术语：占位类型placeholder

Here’s a generic version of the same code:

struct Stack<Element> { 
var items = [Element]() 
mutating func push(_ item: Element) { 
items.append(item) 
}
mutating func pop() -> Element { 
return items.removeLast() 
}
}

Note how the generic version of Stack is essentially the same as the nongeneric version, but with a type parameter called Element instead of an actual type of Int. This type parameter is written within a pair of angle brackets (<Element>) immediately after the structure’s name.

Element defines a placeholder name for a type to be provided later. This future type can be referred to as Element anywhere within the structure’s definition. In this case, Element is used as a placeholder in three places:

To create a property called items, which is initialized with an empty array of values of type Element
To specify that the push(_:) method has a single parameter called item, which must be of type Element
To specify that the value returned by the pop() method will be a value of type Element

https://docs.swift.org/swift-book/LanguageGuide/Generics.html

Generic programming is a style of computer programming in which algorithms are written in terms of types to-be-specified-later that are then instantiated when needed for specific types provided as parameters.

template<typename T>
class List
{
   /* class contents */
};

List<Animal> list_of_animals;
List<Car> list_of_cars;

Above, T is a placeholder for whatever type is specified when the list is created.

https://en.wikipedia.org/wiki/Generic_programming