Flexible box, or Flexbox in short, is a set of properties in CSS introduced in 2009 to provide a new, exceptional layout system. The Flexbox module is identified as a part of the third version of CSS (CSS3).
You are probably already using many of the new properties in CSS3, such as box-shadow, border-radius, background gradients, and so on. However, Flexbox has yet to see the widespread adoption that it deserves. This may be because of all the breaking changes it has suffered over the years, or because it is only partially supported in Internet Explorer 10, or just because Flexbox is a whole ecosystem while previous paradigms have been predominantly based on single, ready-to-go properties.