Flex

flex is a value of the CSS display property. Along with inline-flex, it causes the element it applies to to become a flex container, and the element's children to each become a flex item. The items then participate in flex layout, and all of the properties defined in the CSS flexible box layout module may be applied.

There is also a flex property, which is a shorthand for the flexbox properties flex-grow, flex-shrink and flex-basis. This property is only applicable to flex containers.

In addition <flex> can refer to a flexible length in CSS Grid Layout.