From Halopedia, the Halo wiki
Zen is a school of Mahāyāna Buddhism notable for its emphasis on practice and experiential wisdom—particularly as realized in the form of meditation known as zazen—in, the attainment of awakening. As such, it de-emphasizes both theoretical knowledge and the study of religious texts in favor of direct individual experience of one's own true nature.
UNSC Artificial Intelligence constructs are fascinated by Zen philosophy because of their natural creativity and limited life span. Dr. Catherine Halsey, designer of many of these AIs, is sometimes able to manipulate them by presenting them with Zen philosophies.
Though the SPARTAN-II supersoldiers are never mentioned as being religious, some of them practice Zen exercises to hone their combat skills. Most notably, Linda-058, who is said to enter an "icy cold state of Zen no-thought" whenever she is sniping in combat. Kelly-087 practices Zen arts on a daily basis, including Zen twitch response drills to keep her reflexes sharp.
List of appearances