What is “Orthogonality”?

Question

What does \"orthogonality\" mean when talking about programming languages?

What are some examples of Orthogonality?

Answer 1

Real life examples of orthogonality in programming languages

There are a lot of answers already that explain what orthogonality generally is while specifying some made up examples. E.g. this answer explains it well. I wanted to provide (and gather) some real life examples of orthogonal or non-orthogonal features in programming languages:

Orthogonal: C++20 Modules and Namespaces

On the cppreference-page about the new Modules system in c++20 is written:

Modules are orthogonal to namespaces

In this case they write that modules are orthogonal to namespaces because a statement like import foo will not import the module-namespace related to foo:

import foo;            // foo exports foo::bar()
bar ();                // Error
foo::bar ();           // Ok
using namespace foo;
bar ();                // Ok

(adapted from modules-cppcon2017 slide 9)