What are "Libraries" in unity?

Sorry if i'm in the wrong category but what count as "Libraries" in unity?

 

It generically means any package or suite of code (and possibly assets) generally intended to work together.

Examples: TextMeshPRO, ProBuilder, UnityPurchasing, the OpenCV package, etc.

 

Does
using System.Collections;
using System.Collections.Generic;
using UnityEngine;
count as a library as well?

 

Those are namespaces https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/namespaces/. Many "libraries" use namespaces to organize their code - but there's no requirement that they do so or that a namespace contains code only from a single library, or that a single library contains only code in one namespace. They are orthogonal concerns.

The truth is the word "Library" has many confusing and interrelated meanings.

There's "library" in the "software library" sense - the way Kurt described. https://en.wikipedia.org/wiki/Library_(computing)
There's the Library folder (in the root folder of your project) which is a folder that Unity uses to store compiled scripts and other temporary artifacts that are used for running and building the game.
There's the Library folder that Mac OSX puts your data in when you use Application.persistentDataPath.

 

Thanks guys I think I'm starting to get it!