spontaneous cessation of breath without any conscious effort.
Kumbhaka is of two kinds, viz., Sahita and Kevala. That which is coupled with inhalation and exhalation is termed Sahita. That which is devoid of these, is called Kevala (alone). When you get mastery in Sahita, then you can attempt this Kevala.
In yoga this is known as kevala kumbhaka, automatic, spontaneous retention. The moment your mind ceases to function or is consumed in the point of concentration, automatically the breath must stop because in the brain these two activities are interconnected.
When the breath stops without any effort, when Kevala Kumbhaka comes by itself without any exertion, know that Kundalini Shakti has become active.
deep breathing exercise, Sukha Purvaka (easy comfortable) Pranayama during walking, Pranayama during meditation, Rhythmical breathing, Suryabheda, Ujjayi, Sitkari, Sitali, Bhastrika, Bhramari, Murchha, Plavini, Kevala Kumbhaka, etc.
See also: What is the meaning of Kevala, Kumbhaka, Yoga, Breath, Kumbha?