What is Access Level?

Legal Definition
In computer science and computer programming, access level denotes the set of permissions or restrictions provided to a data type. Reducing access level is an effective method for limiting failure modes, debugging time and overall system complexity. It restricts variable modification to only the methods defined within the interface to the class. Thus, it is incorporated into many fundamental software design patterns. In general, a given object cannot be created, read, updated or deleted by any function without having a sufficient access level.

The two most common access levels are public and private, which denote, respectively; permission across the entire program scope, or permission only within the corresponding class. A third, protected, extends permissions to all subclasses of the corresponding class. Access levels modifiers are commonly used in Java as well as C#, which further provides the internal level. In C++, the only difference between a struct and a class is the default access level, which is private for classes and public for structs.

To illustrate the benefit: consider a public variable which can be accessed from any part of a program. If an error occurs, the culprit could be within any portion of the program, including various sub-dependencies. In a large code base, this leads to thousands of potential sources. Alternatively, consider a private variable. Due to access restrictions, all modifications to its value must occur via functions defined within the class. Therefore, the error is structurally contained within the class. There is often only a single source file for each class, which means debugging only requires evaluation of a single file. With sufficient modularity and minimal access level, large code bases can avoid many challenges associated with complexity.
-- Wikipedia
Legal Definition
Refer to the term system access level for clarification.