Ukemi is the reaction of uke to a technique, especially in the sense of rolling or falling. The word literally means "receiving".
Rolling and falling is practised with and without a partner. The goal is to make it as natural and smooth a movement as possible, in order to reduce the impact.
Particularly soft forms are often called "feather fall"; the Aikido teachers Frank Ostoff and Jan Nevelius Shihan have developed an elaborated curriculum for their practice.
In this video, Donovan Waite Shihan gives an overview of the basic forms for rolling:
The following video shows basic exercises for the forward and backward roll (mae-ukemi and ushiro-ukemi) from the kneeling position (seiza). The exercises are partly influenced by forms practised in Systema and Yoga.
Maung Maung Thant from Myanmar shows progressive featherfall exercises developed from those basic forms:
The following Video shows a tutorial for rolls practised in Parkour. The exercises are well suited for the application in Aikido.