Can you explain what the term 'Single' means in the context of software development?

In the context of software development, the term "Single" is often used as part of the Singleton design pattern. Singleton is a pattern that guarantees a class has only one instance and provides a global point of access to it.

For example, in iOS, Singleton is often used for managers that should be unique within the application, such as a settings manager or session manager:

class SettingsManager {
    static let shared = SettingsManager()
    private init() { }

    var userName: String = ""
}

// Usage
SettingsManager.shared.userName = "User"

Here, shared is the only instance of the class accessible from anywhere in the application.